He knows how you feel about it, he has said he is not willing to change to accommodate those feeling. The next move is yours: Your choices are (a) accept him as he is, or (b) break up with him.
Are you familiar with internet columnist Dan Savage? He says there are two kinds of undesirable characteristics in romantic partners. One kind, the minor but annoying things you can live with but don't really like, he calls "the price of admission." Things like "he leaves his dishes on the counter instead of putting them in the dishwasher," or "she leaves her makeup all over all the surfaces in the bathroom." Things you've asked your partner to change, but for whatever reason the change is not forthcoming. Irksome, but not serious enough to break up over. If you really like this guy and think the relationship is otherwise worth having, then putting up with the way he dresses is going to be the price of admission to this relationship.
The other kind of undesirable characteristic is the Deal Breaker. Things you just plain can't live with. An extreme example would be abuse of any kind.
I think it's clear that your boyfriend isn't going to change his style for you, at least not now. So the question is: Is this a Price of Admission, or is this a Deal Breaker? And only you can decide that.
But note: Constant nagging to change something about yourself that you don't want to change? That's gonna be a Deal Breaker for a lot of people. Just sayin'...
ETA: I see you have already said this is not a deal breaker for you, and you are just asking whether we would want him to change. Honestly? I think he's adorable, he's obviously very young, you say he dresses appropriately when the situation calls for it, and the only issue is the bare feet. I'd totally leave him alone at this point.