well he is just judging the book by its cover..
don't force him!!
let him hate it and when all harry potter is over and he find it really good, then, let him regret it..
but don't destroy your friendship because of harry potter..
friendship doesn't measure with what you want and what u don't want..
be a friend to him..
convince him that he will like it..
and when he discover himself that it is good, let him regret it by his own
cherish your friendship