We are still getting away with sweats during the winter for the time being, but I've found that there are more acceptable (if more expensive) brand-name versions of sweats out there with a drawstring waist and pockets that kind of edge them over into the world of pants a bit; they don't have gathers at the ankles. (DS won't do the athletic mesh or any kind of nylon pants that make noise.) I have found these at Kohl's and other major department stores. (This is where I use the jacket cinch cord stops from REI to make tightening the waist easier. You can also cannibalize old jackets.)
I also wonder - in the winter, do you think your son might wear flannel-lined jeans if they had an elastic waist? I've found them at Wal-Mart, but since they're out of season you may have to check on ebay. Carhartt is one brand - I'd find a used pair online to see if they will work first (DS uses "adjustable waist" jeans, which he needs anyway as he's not traditionally-sized. Since they use a buttoned-in elastic band to adjust, he uses them without a belt as though they have an elastic waist.)
Also, we tried many belts, but until it broke, DS had good success with a plain web belt with a magnetic buckle, and was willing to wear jeans and pants that needed a belt; the key was keeping the pants waist large enough that he didn't have to unbutton/unzip and button/zip. We adjusted it once to the correct length, and all he had to do was thread it and put the two pieces together. I think I got that at Kohl's as well, but they should be everywhere.
Good luck!