You can claim him as a dependent, on your tax return, and receive a $500 "other dependent" credit. Because he is over 16, you do not get the more generous "Child tax credit" (up to $2000), even though he is disabled. That $500 tax credit is "non refundable". That is, It can only be used to reduce your tax liability. If you tax is already at 0, you don't get any of the $500 credit.
Only children under 17 (essentially those that who qualify for the Child tax credit and not the other dependent credit) are eligible for the $500 stimulus.
