Ah, the electric guitar plugged in and loud is a very exciting thing, especially for a 14 year old boy. Headphones are dodgy if he plays loud through them, he could damage his ears a lot more easily.
100 watts is very loud, no need unless he's gigging and even then not really, thats what microphones and PA's are for.
What about through a PC with decent monitors? The Line 6 Toneport sounds great and gets some great sounds that way. I generally just practice unplugged though, but I get my volume fix at weekly band rehearsals in a rehearsal room.
Otherwise just tell him if he does not turn it down you'll take his amp off him and burn it, simple.