Dunno about NORMAL SS disability - assuming he never worked like most disabled people - max entitlement without a full work record was ~$760/mo (I think $753/mo specifically - thrown out all my disability update papers for my numbers, but was always a blurb in the annual COLA paperwork telling me what inflation adjusted my ~$1200/mo I had been getting during my sick times to) last time I checked. 21 x $15 = $315/mo.
Smells like a different assistance program to me. Or he's a basement dweller not paying for food and rent that can afford to spend half of his income on a game. Or he sees it as an investment into potential income that he can dodge income tracking on. (Which honestly, if someone cared enough to dox him seems like something that would be trivial to audit these days and I'm sure would ruin his day in a hurry)