By the SSA definition, "substantial gainful activity" is a dollar amount. So by their definition, if you are capable of making more than
$1130 in 2016, you shouldn't have SSDI.
There are two things at play here. One is the ability to engage in substantial gainful activity (SGA), which is usually gauged by whether the SSA finds that you're able to hold down a full-time job that exists in the national economy, previously from an ancient book called the Dictionary of Occupational Titles. If you could theoretically, as established by the testimony of a vocational expert or some other evidence, hold down some job in the canonical list, you weren't disabled.
The
ability to engage in SGA is more of an abstract concept than an actual thing.
Actually engaging in SGA, though, is another, and when you
earn a certain amount of money, you are presumed to be engaging in SGA.
If you have some conditions, codified in the Listing of Impairments, you are presumed to be unable to engage in SGA. You have no legal obligation at that point to try to prove this presumption wrong. You can simply not work or quit work and collect disability. Some of these are obvious, like amputated limbs. Others, like mental disabilities, usually require establishing first what are called the A criteria, objectively establishing the existence of the disorder, but then other criteria, often called the B or C criteria, must also be satisfied to establish that not only does the condition exist, but that objective medical evidence establishes that the condition has consequences that result in a presumption of disability.
Autism has A and B criteria:
https://www.ssa.gov/disability/professionals/bluebook/12.00-MentalDisorders-Adult.htm#12_10
I am assuming he met the criteria for that, which would establish as a legal matter a presumption of inability to engage in substantial gainful activity.
So to disqualify himself, he would actually have to engage in such activity and
earn income. Unearned income doesn't count against this particular disqualifier.
Ironically, this would mean that he could conceivably be penalized for actually doing creative stuff like making medallions and selling them, while people could send him free money all day long.