Think of a way to prevent addiction before you buy this game for him
Yeltsu is right, the game itself doesn't contain anything that would be harmful for a 10 yr old*, but the players, omg... Sometimes they are so stupid, cursing in the main chat, flaming each other and stuff... There's the ignore list, though, so he can avoid looking at such conversations easily.
The main problem with this game is that it takes very much time, and if someone has it, he will spend it with playing, no doubt.
*There's a 12+ sign on the box, though! Maybe the violence is too much for such a young kid? I remember I was taken aback by one of the starting quests for Druids, where I had to kill some kind of animals, I felt so sorry for them - I believe how a much younger (=more sensitive) kid would take it even more seriously.