My first child did this at about the same age. Not to sound mean, but he's old enough to be told firmly NO and hold him down for a second when he squirms. Being commanding about this will only make things easier for you later when he's bigger and even harder to control. Explain that he must hold still and that it will take longer if he doesn't. Generally with a bit of firm will they get over it pretty quickly. Also, try to give him a place to squirm and turn like he wants to, put him on the floor and tickle him while he does it, so he knows there is a time and a palcefor that sort of thing.
Good luck!
Also, after reading other responses, as a foster to some of the most horrendous kids with golden hearts, please don't cave to him! As they grow they test more and more buttons, and while some battles aren't worth the fight, teaching them to obey you physically in this fashion is VERY important. Give him a special toy, sure, but don't cater to him entirely. Standing up changes leave creases in their skin and make it very difficult to get a proper clean and fit with a diaper. I guess using cloth diapers, standing changes just aren't an option for me! Does your changing station have one of those straps? I've used those with a stuffed animal (put the stuffed animal under the strap with him so it's something to focus on other then what you're doing to his bottom) with success when I've had persnickity fosters who wanted to roll over. Aain, good luck!