Your son is taking quite a
high dose of Concerta. There is no reason
to expect that he will not continue to have suppressed appetite. It's not so much an issue of how long he has been on medication, but rather an issue of finding the best medication for him (i.e., the medication that proves to be clinically effective without untoward side effects). Stimulant medications (e.g., Concerta, Ritalin, Adderall, Metadate, etc.) all have the potential to suppress appetite.