Will he be playing on PC or a console?
There are some names, but as Johnny_Big pointed out, there are many variations... If on PC, then there are some stuff to consider too, like PC specs...
For PC, if he likes Diablo-like games, Diablo 2 + Expansion shouldn't cost much, though it's a bit dated. Even if you consider getting a new mouse after all the clickety-clicking...
On another sub-genre, Vampire: Bloodlines is a great game, and as close to a RPG as possible, since it's based on Vampire the Masquerade, if a bit on the heavy side (both in computer requirements and content - how old is your bro? I wouldn't want a 10 years old seeing a vampire eating human flesh...)