I'm not sure if he has informal accommodations or a 504 plan, but failing to inform a substitute teacher about accommodations in a 504 plan is illegal (as well as very common). The school will give you a song-and-dance about your DS' privacy if you ask about this. I suspect that your best bet is to include in the 504 as one of the listed accommodations that everyone who works with your DS, including volunteers and subs, will be informed of his 504 plan and accommodations.