J.W.
This happened to me and my son as well. I swear I got to be the fastest diaper changer in the west! Lol. I just had to be really fast and try to pin him down as well as I could while I changed him. This phase lasted for about 2 months, and then suddenly, it stopped. I used to say things to him, like "stay still" and "no,no" and whatever else I could think of. Try giving him new toys that he hasn't seen before, or even things that aren't toys, but are ok to play with. We didn't have the strap on the changing pad, I think that would have been very helpful. If you can, try that. Good luck!