There are many excellent books around like "What's happening to me?" (don't remember the author) about puberty and all those things. Ask at a good bookstore or library. In the meantime reassure him that it is normal and even a Good Thing.
My two year old gets erections all the time! He has ever since he realized that hands can go down there ! I guess with a kid that young, it isn't really called an erection but it sure looks like one!
I think the last poster is right. Assure him that it is normal and perhaps get a book that he can understand at his level.