OK, I *think* it is either/or. He should get the largest of the amounts, I can't see anywhere online where he would get a combination.
So if a disabled adult child is accessing benefits from his father who receives $2000/mo in SSID. The child would get $1000/mos- considerably higher than SSI.
If accessing from his mother who makes $1000/mo he could get $500 in which SSI is more.
Please correct if I am wrong.