Your friend, I'm assuming, is an adult. Just because he is a single man doesn't mean that he can't clean up after himself or that he doesn't know how. He is clearly choosing to keep his house messy and that is his prerogative. Yes, he apologizes for the mess when you come around which — I'm guessing — is because he realizes that the way he lives isn't for everyone (especially not for you) but that doesn't mean he wants to change things. It doesn't seem like his messy house has been much of a hinderance to his hosting or I'm guessing he would have done something about it by now.
Maybe he does want to paint his place and I'm sure he would appreciate your help, but don't expect him to become a clean convert. You can't just do a big clean for him and make him see the errors of his messy ways and watching his home revert back to messiness (after you've cleaned it) will be pretty frustrating for you. I'd leave well enough alone and put your efforts towards your own home where you can appreciate the results.