Often enough, as a girl, you will always receive proposals...
and the way you respond to it depends much on both your character.
That explains why he is like that. For a fact, perhaps he like you
too much until he is deeply madly truly in love with you he will
not accept no as an answer (Dumb and Dumberer, the movie)...
Have you approach him telling him NO directly? If you have and he
still responds this way, it is important to bring in the third
party to explain things to him. Try to take it up the level until
he fully understands your position. As in after your friend
explained the situation to him, and he still acts the same towards
you, take it up another level by recruiting your parents in...