O-o-ooohhh, okay. You've made the situation so much clearer now. Thank you. Yeah, he seems extremely immature. I wonder what your GF gets out of her relationship with him, if they aren't sexual and she can see why he's so annoying to you, LOL.
It doesn't seem anywhere remotely a situation where the three of you should move in together. That is never anything to take lightly, even if one is used to having a ton of people around. It's your space and you have every right to some peace and quiet and alone time!
How long have they been seeing each other? With someone like that, who is so immature, pouty, a bit of a drama junkie (seems to me), and to some degree just taking advantage of your good nature, I would wait until they've been solidly together for two years before even considering living with him. He needs to show more of a track record of responsibility -- financially and otherwise. And until that happens, I would -- if I were you -- ask for certain days and times that he not come over at all, so you can nourish your time with your GF and not feel invaded. I don't think it's unreasonable to ask for two whole days each week to be set aside for just the two of you.
Just my two cents (but I think this is actually brilliant of me!)...