It is difficult to find onesies in larger sizes. I know Gerber did a run of size three and four maybe two years ago but I haven't looked for them since. To get them cheaply is another thing alltogether. A lot of websites will say they have them but when you go to put them in the cart and check out they say out of inventory.

You don't need that anyway. Buy him a pair of footed jammies that is one size too big for him and put them on him backwards. You don't have to cut off the feet... just turn the feet forward and put them in the way you normally do. Do both feet then put the arms on and then zipper up the back. He would just live in a pair of backwards jammies while he was in my house. I can take a picture of a kid in one today to show you and put it on my Facebook page if you would like to see how EASY it is. If he wants to do it badly enough a onesie won't deter him. It's just a couple of inches down to the thigh seam of a onesie. He will be able to easily go down the side and lift up the butt cheek of the onesie. It will be a bit more work than going down the back of the pants but he'll easily get to the target area with little effort. The only way I would use is onesie is OVER his clothes meaning over his shirt and over his pants. I've still had kids that can get to themselves thru the side with that but it's much harder. If you do that trick put the onesie on backwards too so the snaps are in the back. Footed jammie trick works way better.