One thing about senior year - if his high school is anything like my son's - the courses that you take in your senior year are often easier and in total less demanding than the Junior year courses (assuming you have been on an aggressive academic track). Acaemically, he may actually be better off and far less bored at USC.
And I do think that his situation - being a very young member of the class of 2007 - is something that probably played an important role in this decision - as Dad04 said - Rookie Ball at 17 could be a very scary idea. USC is close to home - and 3 years there will still make him a very young draftee...