Read our books together with blankets tucked up around us. Of course, that's generally her idea, but it might be possible to suggest it to a 2 year old and have it work.
If he's willing to try various activities, stringing beads, moving beans between containers with tweezers, connect four (just sticking the checkers in, not trying to play the rules), dropping q-tips into holes punched into the top of coffee cans--fine motor stuff that needs a lot of concentration lends itself well to sitting quietly and concentrating.
If he needs convincing:
Could he do something messy that's normally not in the cards? E.g. fingerpainting, corn starch and water in a bowl (ooblick), anything where he can sit in front of a bowl of glop and work his hands instead of his body.
Since it's a breathing treatment, I'm guessing that trying to wear him out before the treatment would be counterproductive?
Really soothing music and a video that's entirely of something he adores? Or, since this is a medically necessary situation, see if something with bright flashing colors is more hypnotizing.
Can his legs wiggle and the treatment still work? Could you play a game where you hold stuff up and he kicks it with his feet? And if he just needs to be still, but the quiet part isn't as important, could he sit on a wheeled chair and you push him around and spin him around? (Obviously, not going to work if the nebulizer's plugged into the wall.)
Maybe try starting kid yoga with him? It might not help quickly but could be helpful over time.
