Tell him politely as Rappy said, but be assertive.
If he still just doesn't latch on, try to break contact. It may be hard if you want to be friends, but he needs to understand that if you don't want to and can't have a relationship, then neither of you can. If you haven't explained the full situation of you being 20 and him under aged and the fact a relationship would be illegal, yada yada, do so. Don't be scared about hurting him, seeing you are not in a relationship he will hopefully not be too attached. THOUGH, in saying that, and as horrible as it probably sounded, be weary of his emotions. Nobody likes hurting others but compare the potential of jail versus someone with hurt feelings. Big difference.
In the end though, if it continues and you really must stop it, blocking him will most likely help.