Amps like Line 6 Spider series (I own one and it's great) or Roland Cube (I still have my Roland 15 watt, 14 years and still working. It's not a cube model but Roland is a reliable company) are probably good amps to look into for your son.
Personnaly, I never crank an amp (always had combo amps) passed 8. It's just an habit I've developed of the years. If I need more than what my amp can handle at 8, it's time to look for a bigger one. Eventho they go up to 10 I never feel easy cranking them up. I don't know if it has anything to do with it but I never blew up an amp :)
As for what can damage a speaker, unfortunatly I'm not a gearhead so I can't really shed any lights on that part.