Tell him about how it gets there. And then have him throw it in the toilet and wave good-bye to it. Have him stand up. This can help him feel more powerful than lying down. Make sure his needs are satisfied.
Timing is everything.
Don't try to change his diaper when you are feeling rushed or when he is in the middle of a favorite activity. Give him a choice. My son and I even made it a game. Sometimes, if possible, I would do it. Have supplies ready before you get to the changing table.