My son is a HS senior who had Tommy John Surgery in Sept '08. His surgeon was very upfront about the peaks & valleys he'd experience (3 steps forward, 2 steps back). Going thru rehab would be hard for anyone, much less for an intensely competitive teen who wants the ball!
My son has been cleared to DH this spring -- he was bummed about missing his senior year on the mound. However, things are looking up. He signed with a junior college that's committed to his rehab, plus he's helping his team with his bat. The goal is for him to start pitching this fall and be "ready to go" next spring (18 mos mark).
Note that we followed the progress of an area pitcher (2008 draft signee) who had the surgery as a HS sophomore... took him one year to "rehab" and another year to get "the feel" back. Career-wise, a short-term sacrifice for long-term gains.
Hope that helps. Good luck to you & your son!