So, I have a group of entirely new people, and most of them have good characters, and have fleshed out their backstory. I had the first session yesterday, and I was asking some questions. One of them was: “do you want more roleplay or more combat heavy sessions?” Most of them answered roleplay, which is fine. But one guy answered combat. That’s fine, I can find some balance to appease most people. Then I asked about powergaming. Most said that they wouldn’t do it, one even saying it defeats the purpose of the game, except that one guy. He said that powergaming is the best way to ensure we win. His character is already min/maxed out, being built specifically to bolster our team’s weaknesses. He has nearly no interest in making a fleshed out backstory, however. How I can handle this better?

Have you seen any Samuel L Jackson movie?  He is basically the same guy in every one.  Tough,  no nonsense, don't take crap from anyone.  He nevers plays the weakling, or the super smart nerd, or the love interest.  There's nothing wrong with that - he plays the bad-ass thug-like (if not actual thug) guy REALLY WELL. 

Everyone plays a role.   Your player has picked a role too, he just may not be aware of it.  On the bright side he will be consistent.  He's not going to suddenly stop trying to win because he is playing the classic  "win at all costs man".    That may be the only role he can play.   It might be him in real life.  

Let him get mad at the other players for wasting time with the inn.   Never break the scene and call him out for it, instead insist that his CHARACTER is complaining, no him.   

Just be sure to give him so combat and let him shine.  Make sure he is happy at least some of the time.

The main problem I see is the concept of "we win." If he means the players beat the DM, you have a potential very serious problem on your hands, because D&D should not be the players vs. the DM.

@biowozard: No, he meant the campaign. I am currently trying to tell him that it is not a win/lose scenario, but a storytelling experience.

@mog_dracov: I guess, and there will be combat at least every other session, I will try to have it every session though. It’s just gonna get complicated when everyone but him gets wrecked by a boss or something, and when it’s his turn he kills it instantly. Hey at least he has 1 flaw..... he has a one in char. (We rolled dice for our stats)

Well you are not going to change a powergamer. You can either live with his tendencies and try to give him what he wants without disrupting things for the rest of the party, or you're going to have to stop playing with him.

It seems like he’s aware of it, at least. Maybe have a talk with him and point out that the rest of the group isn’t on the same page, and that he should manage his expectations of how the others will play. And have a similar talk with the rest of the group. Let everyone know they will need to make some compromises. Just head it off before people start to get upset. 
How’d he get a 1? Even the strictest method of rolling is 3d6. A 1 should be impossible.

@Xalthu:  It's possible they rolled a straight d20 and got the worst possible outcome there.

@OP:  It's possible you could find some compromise for the table and find a way to make this work, but if I'm fully honest it sounds like the odds are stacked against that happening.  It's possible you may need to tell this player (gently) that your table might not be compatible for his playstyle and that he might be better served trying to find one more in line with his expectations.

Powergaming in 5e is almost impossible, thanks to the rules. Even if you do your best to optimize you're still well within reach of non-optimized characters. Unless your other players actively try to make their PCs incompetent you probably won't have a balancing issue.

If Cha is his dump stat that's even better. He doesn't sound too interested in roleplaying, so your other players will shine in RP scenes and he will shine in combat scenes. If you find the right balance between those two, your group will be fine. :-)

I would also give him some time. I never write backstories for my characters before I've played them for at least two or three evenings either. But when I get a feeling for how the world and the NPCs and other PCs work I start writing one as well.

I would be worried about a serious Murder Hobo problem.  You may have to DM two campaigns and find him some friends.

Naaahhhh, the rest in my party will not allow his murderhoboing. Especially since 2 of them are lawful good....

Did he say he wanted to run a murderhobo character?  Because powergaming and murderhoboing are not the same thing.

It sounds like the conversation you had with your group was a conversation between the intent and desire of the players, not the player characters, so the question of "can this character be inserted into a campaign with these other characters" isn't really what's at stake for address--the RP of a combat-focused character is just as likely to succeed or fail as any other character (that really only requires the desire to integrate). The true focal point then becomes a matter of compromise--as it stands your pace is set to be stacked more heavily on RP; how much time spent table-side outside of combat will said player be able to commit their patience/endurance to? If that ratio is too off to be enriching for that player, how far is the rest of the group willing to compromise? Is it primarily a gamer-group or a knit friend-group? Each have their challenges and payoffs for sure, but these situations can get tricky in friend groups. If the balance doesn't work but it's a friend group consider alternatives to keep them involved without having to commit as many hours into the story; perhaps they're one of the other characters retired warrior-fathers who is coerced out to protect their kin when the danger gets too tough (they could even have a cooler badass weapon and be a higher level at that point)? Perhaps they play a rotating cast of wandering mercenaries to keep them on their toes and engaged in knowing what abilities they have access to. This way if you played say once a week, you could invite them every other week or once a month so the FOMO is deterred and everyone has fun without dissension at the table. Of course on the other hand of that though, if everyone is patient and cool with sitting around for hours with their friends in wait for their occasional moment to shine, bless 'em.