I actually came out of lurking to make this post. While I don't receive SSI, my roommate does, and he receives it via check. A lot of people still do not have the debit cards and direct deposits.
The checks are being mailed out all this month, so maybe your son is at the end of the list. The official site does state to call them if it does not arrive by June 4th. I hope you have no need to do so.
Here is more information, as well as the number to call should you have a problem. Hope it helps.
Well, there was supposed to be the link. I have under ten posts. If you go to the ssa website and click on the retirees section it has a great faq on all of the ssi related stimulus refunds.