You really can't force him to accept your apology, however if you really want to keep trying, ask him why he is upset with you, and try to understand what you can do to show him that you care about his concerns. Try to understand his point of view. Sometimes friendships run their course and end eventually end, so if things don't work out, it may just be not worth forcefully salvaging.
Explanation:
This is the best advice I can give you. I've been in similar situations, and honestly, have just accepted it. If someone doesn't want to be my friend, that's their loss.
