I have twin 2.5 yr old boys and I have had the same problem. It does get better but i still have fights many times when it comes to diaper changes. I change them on the floor and have to physically restrain their legs - I put my legs on top of theirs. I sometimes swat on the leg, as well, because sometimes that is all you can do. I don't hit hard, just enough to get their attention. My boys have the bodies of 4 yr olds, so it's even harder, and I do understand. If the diaper is not too bad you can try time outs to try to calm him. Also I have tried turning on the TV to calm them and sometimes that works. I think it is just a control issue. Good luck, and do not let him kick you in the stomach! I can tell you that holding and talking and trying to calm them down really does not work well; I just have to go ahead and hold them down to get it done.