I'm developing a platonic relationship with a guy who has a gf but I kind of like him.. Advice?

Well this might be really hard advice but I really think that if you really like him then you shouldn't ruin what he's got going with this girl. I know that it will be really hard and you may not want to but if you tell him you might ruin your friendship and make him feel guilty. And if he really loves this girl he would tell her and she will secretly hate you and that'd make things bad too. All you can do is be his friend and want him to be happy. And who knows if he breaks it off with this chick you'll be there ;) Sorry if I'm wrong which is very possible but I hope it all works out, I wish you all the best and good luck