You can be polite and truthful at the same time,but really.....your going to loose him as a friend when he does finally get the point.Your "worldy"(i hate that word..pfffffttt) and they can't have association with the world as in friendship.
When it comes down to it..he really ins't your "friend",your a book study and hours for feild service.
Sorry if I sound harsh,I don't want to come across as a meanie.
***hugs***
~Heather