I'm willing to guess that most of his subscribers -- like most people in this thread -- don't know enough ASL to realize that he's not really that good. He's got some grasp on the language, but interpreting songs in sign is as much an artistic performance as singing is, and from what I can tell all he's doing is jumbling his way through rap lyrics in pidgin.
I don't know why hearing people get so much credit for doing this (and doing it relatively poorly) when plenty of Deaf people out there have been doing awesome music video interpretations -- and sometimes even original songs -- for ages. If you really like the concept of signed songs and want to see a few people who actually excel at it, check out the following:
Signmark (primarily ISL, not ASL)
and Amber Galloway Gallego who isn't herself deaf, but does a ton of performance 'terping