whatamess - Great thoughts here so far. I just wanted to chime in and say that I understand - on the one hand, you know you need to be flexible with issues that aren't "big" enough. Yet, here you are with a 12yo who refuses to wear underwear, and it would seem that this could rise to the level of being a "big" issue as he hits the teen years.
Our 9yo son has Pervasive Developmental Disorder (PDD) also, and we have similar problems. He wets the bed a lot, and we wish he would wear those overnighties to bed so that his bedding would be more protected. He refuses in a big way - can't stand how they feel, etc., etc. We tried a lot of different things, and nothing has worked . . . yet, anyway. I do think it's important for you to get to the root of the problem - why your son doesn't want to wear underwear. If it's because of sweating, then perhaps it is a sensory issue, or could it be something else?
I read somewhere once that kids with autism may have a negative experience with something, and especially if its their initial experience, they'll continue to harbor that same perspective every single time they encounter a new similar situation. In a sense, they have to "unlearn" their previous habitual response.
Our son has a thing about his bedding. He hates sheets and doesn't like regular bed-size blankets either. There was one fuzzy blanket he liked, but it was small, and as he grew, it became inadequate for keeping him warm at night. So, I went out and bought a new blanket (twin size) for his bed that had the same fuzzy feel. It was also time to replace the fitted sheet on his bed. I knew he wouldn't want a flat sheet, so I had no plans to use one. Anyway, the first night, I made his bed, and he cried about it. I found him with his same old teeny weeny blanket on in the morning and all of the new bedding ripped off the bed and piled on the floor. The same happened night after night. Finally, one night, he left his new bedding and blanket on the bed all night. Then, he started liking his new blanket so much, he'd want to drag it around the house all day long. He's still doing that, and the new awful blanket has become the new best blanket. I don't know why, but it's almost like he needed time to adjust and get over his preconceived ideas about the new bedding. Sometimes kids with autism embrace an initial fear reaction to new things. Or, they may suddenly become averse to something they used to accept. I suspect all of this is rooted in sensory processing, but maybe also the fear of change or a need for control of their environment.
Maybe your son won't compromise on this for awhile. But, keep the door open through encouragement and the willingness to help him find something comfortable. He will probably change his mind at some point.
(((hugs)))
Weary