I agree with Dr. Hagan's advice (above). However, if there is truly nothing that the strabismus specialists can do, you might attempt a trial of vision therapy if your son is really motivated. (It works best for eyes which turn outward--toward the temple.) Vision therapy did help me to some degree (I stopped after surgery eliminated my double vision), although I found the computerized home exercises very frustrating. (I failed an initial task more than 100 times before finally succeeding.) I'd recommend that you set short-term goals and insist on progress.
No vision therapy is not helpful for this and, when done by optometrists, is inordinately expensive. I suggest you get a second opinon from another strabismus Eye MD ophthalmologist.
JCJH MD