I am a DM and one of my players is currently playing a fighter. Recently due to how the game went it's course, the fighter became more religious and eventually he decided he want's to become a paladin. Sadly he dumped charisma when he made a character because he didn't think he needed it. but due to character growth he now does.
His charisma is currently a 7 and just giving him 6 points of charisma seems cheap and also not fair to the other players, especially since we use point buy. (modified)
I could do it that he loses some stats different aspects of his character to make up for it but that will probably also hurt his character a lot.

How would you guys solve this problem? I do believe of letting him multi class into paladin would be awesome and make a great story. But I just don't know how make the character work in terms of mechanics and keeping things fair for everyone.

I would just let the player to use the same point buy system to create new array of ability scores, which would fit this fighter/paladin multiclass.

Just turn the restriction off and let him multiclass. No headaches, then. He can then focus on spells and features that don't rely on spell attacks or saving throws. You don't need charisma to smite.

None of the Base Class features rely on Charisma, bar two that remain beneficial anyway. 

The same can be said for many Oath features, where even those that offer +Cha bonuses often have a "minimum +1" part. So they remain useful anyway. Some features do have force saving throws and those should be avoided since the DC would be too low. 

There's spells you can take that don't require spell attack rolls or saving throws such as: Bless, Ceremony, Detect Evil and Good, Detect Magic, Detect Poison and Disease, Divine Favour, Heroism, Protection From Evil and Good, Purify Food and Drink, Shield of Faith, Aid, Find Steed, Lesser Restoration, Locate Object, Magic Weapon, Protection From Poison, Aura of Vitality, Create Food and Water.... And more. I'm getting bored listing them all, there's so many!

So, you can still have a very effective Mary-Sue , I mean Paladin even with a low Charisma. 

Now this is completely not in the rules, and it would require both you and the player(s) to take on some additional responsibilities; but I think there is a unique situation here that can add layer of fun play.  

Research D&D Beyond and maybe some outside source materials for magic items that when activated raise the character's CHA score to a 20 (you will probably have to homebrew the item so it has the right amount of charges and time uses to fit within the game you are running).  Now when the player levels up to their Paladin 1-level have their god bestow this magical item on them. When the character doesn't have this item activated their CHA score remains as is (based on the character's level and development) and if less than a 13 then the character doesn't have access to their Paladin's abilities.  I would argue the can retain the Paladin hit die since it is the same as a Fighter's and the Fighting Style selected at level 2 can be allowed; but the divine or magical features that come with the Paladin are not available to the character.  Only when they activate this item provided by their chosen god and their CHA score increase above 13 do they achieve these powers.  

Again, I know this is not in the rules.  I just proposing this a potential game element.  Now the player(s) can be engaged with challenges to ensure they have access to the magical item and there is a management requirement involved as well. 

For a long term solution, the player can invest Ability Score Improvements and maybe find a Tome of Leadership and Influence to raise their score to a 13 and thus allowing them to access their Paladin abilities all the time.  But as a short term solution, the above option allows for limited and situational play. And if the player opts to increase their CHA to meet the multiclass requirement, the magical item still has a huge benefit during encounters because it raises their spellcasting modifier to 20. 

This will take some work to build out, and commitment form the table.  But is a homebrew option that is unique and could add elements to the game play. 

Why does he have to become a paladin? What’s wrong with just being a really religious fighter who is favored by his god? Some things can be handled purely through role play; not every character development needs to involve a game mechanic.
If you really want, you can always give him a boon that mimics a paladin power or two like divine sense and lay on hands. That could add some nice flavor without making his character crazy powerful. Or give him a level 1 smite pb/long rest. Or whatever might be an appropriate gift from his god. 

Thank you all for the solutions offered, I am gonna talk with my player and the rest of the table what they will find a good solution. I like the idea that he is just gonna be a negative charisma paladin that is slowly getting buffs to his charisma over time to make it at least something like 14 but this will have to happen over the course of maybe a year IRL time.

The player in question is quite new and doesn't understand all the mechanics yet and luckily he  isn't a marry sue. As for why he wants to be a paladin as a player. He first thought it would be really cool to be this awesome fighter and hacking and slashing at everything and dealing tons of damage, but he looked at the rest of the party (which are almost all magic casters) and realised that already he can't do very much outside of combat. I already homebrewed and adjusted some abilities to be more versatile and giving the martial characters more options but sadly they will probably never be on par with casters.

Again I would like to thank you all for the great ideas and taking the time to replay.

How about letting him multiclass into a cleric?

The problem is that he needs 6 points to qualify for paladin. If it was 2 or three he would have a chance to gain those ASI soon.

You do not want to penalize the player but just letting them rewrite the character and keep going does penalize the other players in a way.

Quote from TheGnome5786 >>

The problem is that he needs 6 points to qualify for paladin. If it was 2 or three he would have a chance to gain those ASI soon.

 
Only by RAW. As a DM this can be handwaived away. Doing so does not affect game balance in any way, given that you can be a very effective Paladin even with Cha being a dump stat. 

I just want to clarify my Mary Sue comment was regarding the class not the player.

Paladins are basically the Mary Sue class. They're very good at absolutely everything: they deal huge damage, can tank, can cast damaging spells, can cast control spells, can cast utility spells, have very strong base class features, have strong subclass features and their only slight weak point is range. A weak point soon overcome with access to mounts via the paladin only spell Find Steed letting them close distance easily without using their own movement. Eventually they get Find Greater Steed for a flying mount, completely removing their only weakness entirely.

And they are very easily multiclassed if wanted. In fact the Sorcadin, the Sorcerer-Paladin, has the highest consistent damage of anything else capable of nearly soloing dragons if they get a little lucky. 

Yes, I am still salty that my dragon got wrecked by a sorcadin in a single turn - no matter how much I tried to redo the math of their critical divine smite. I hate paladins.

Quote from Cyb3rM1nd >>

Quote from TheGnome5786 >>

The problem is that he needs 6 points to qualify for paladin. If it was 2 or three he would have a chance to gain those ASI soon.

 
Only by RAW. As a DM this can be handwaived away. Doing so does not affect game balance in any way, given that you can be a very effective Paladin even with Cha being a dump stat. 

 Give every player the same chance and no class limits and it would be fair.