Give him something to distract him. Small books worked well with my kids, but a small toy or a mobile above the change table might help.

Get everything ready nearby, distract bub with a toy or song. Hold their ankles if need be.

Im a foster carer and you be surprised how many bubs kick their legs while changing. Its a nightmare when they have done a poo and kicking their little legs. haha. I give my little one something to look at or a toy that usually helps.

I can feel you my son did the same and as many other wiggly babies does it, just give him something he likes for example any toy to distract him while you change his nappies.

I always give them something to play with

Practice, basically. Have everything ready (eg. new nappy, wipes, bag for old nappy, cream and anything else you use) and use one hand for holding his legs and the other for cleaning. I used to hold the legs by gently placing my hand around the ankles and lifting them so bum is off the ground, and the other hand cleaned and put a new nappy under, which bub was then lowered onto again. It just takes practice and doing whatever works for you. It sure can be a mission haha. Good luck!