If he has a girlfriend then it’s a no go. Even if his girlfriend isn’t a nice person, he needs to see that and needs to end it, not cheat on her with someone else, and you don’t want to start a relationship with cheating. Once a cheater always a cheater and you don’t want to date a cheater.
As for your supervisor, you don’t have to like him that way, you can just be friends with him. If something grows in the future, then you can deal with that as it comes. For now just keep it as friends unless something changes with your feelings