"This is all he does" - yeah, its probably his special interest, Autistic people have what's known as a special interest, which kinda speaks for itself.
and if so, that may be why he's so good at it. some can play a piano solo after hearing a piece of music only once, some play (and are very good at) computer games.
i have 4 alarm clocks (cos i sleep thru 1) all set to the same time
1) bedside alarm
2) bedside alarm number two
3) cell phone
4) MacbookPro (With Alarm Clock 2 app)