I don't remember any of my children fighting me, but I did have a grandson and a granddaughter from different fathers who couldn't stop squirming and were unwilling to go along with getting changed. And that's just it. Your child is unwilling to lay there. With both grandchildren, with pee it's no big deal and with poop, you have to be a little more careful. I'd tried to make diaper changing as fun as possible, allowing them to help bring wipees and diaper to the changing area. With poop, I'd sing songs, play games, ask them questions, while firmly grabbing the legs together in one hand and quickly cleaning the behind area and legs. If they flipped that was ok, i just continued to clean around them as they squirmed away, all the while telling them what I was doing why I was doing it. "Need to get clean," "ooohhh, stinky," using baby language, ugh, and stuff like that. Making potty changing fun. They exasperated their mothers but, I felt more comfortable handling this type of behavior. the children don't realize that they have to sit still for this. My one grandchild now three, is a homebody, likes playing video games and watching cartoons, whereas his brother who almost 2, tells you he has gone in his pants, brings wipees, and his diaper. He is ready for potty training, but has trouble sitting still yet. From the moment he wakes up, and puts on his shoes, he wants to go out into the world, strapped in his car seat. Totally the opposite of each other.
Just go with it, don't fight it. Just have everything in hand to prevent big messes. Play with him and love him.