So, I'm back with another question regarding power-gaming from my munchkin hexblade player, who keeps asking for an expanded polymorph list, and has discovered the option to polymorph into half-dragon beasts, because they still remain beasts.

Unlike with wild shape, polymoprph doesn't state anywhere that you need to have seen the creature before to transform into it, pretty stupid if you ask me. I personally don't want to allow this; one, because it's such a powergamey request. two, I can't really think of a reason that dragons in my campaign would be going around diddling creatures like giant apes, and T-rex. Three; dinosaurs aren't alive, and he knows of them through "history" and there may not even be any recorded information about the existence of half-dragon bullshit.

I unfortunately have come to a point over the last couple months where I am more often than not denying his requests for more power and abilities and additional homebrew mechanics/expanded lists and spells. It's my homebrew game, but I allow what I want, and I will not ruin the balance of my game. I'm not concerned that it's overpowered. because ACs are usually pretty damn low anyway, I just can't make story/worldbuilding sense of it.

Unfortunately this half-dragon stuff is canon and RAW, so I need to think of a valid reason not to allow it, or with higher limitations. Am I being unreasonable with the reasons above? Do others allow this?

I'm with you that the Polymorph spell doesn't magically grant a PC intimate knowledge of the entire Monster Manual, regardless of what their character would actually know. I'd say it's safe to ignore RAW and stick to the Wildshape rule that the PC needs to actually see the creature they're transforming into at least once in their life. I'd apply that same rule to half-dragon beasts... if you have the players encounter a Half-Dragon moose or something, then that player could transform into a half-dragon moose, but I don't think it would make sense to have the player be able to just decide that they're going to have dragon powers on top of being a giant eagle.

Your game.  Your world.  Your rules.  That sums up why you don't NEED to agree to anything he asks for.  As well, you can see he is trying to meta, which you can/should shut down asap.

To eliminate further discussion/debate over the polymorph, give him a list.  THIS is what you can poly into, and nothing else.  No, the list won't be amended and no, your specific requests, since not on the list are not allowed.  I am FOR allowing players input and bumps to skills, spells and more, but ONLY if it isn't going to unbalance things, OR come off as simply silly for the setting.  If it doesn't fit lore-wise with the world I have created, it is out.  Period.  There is no lengthy debate....out means out.

Work with players who are trying to work with you.  Close the door when they start taking it too far.

I do house rule that the max CR for polymorph (and similar spells) is the spell level, not the target level.