On the surface this question makes no sense.
generally speaking there isn't a reason he shouldn't be helped. One however doesn't just slip in an implant. A comnplete examination must be performed to determine why the facial deformity exists. This may include specialized X-rays, dental examination, referral to an oral surgeon, physical examination, perhaps genetic counseling, and maybe even other referrals.
A weak chin must be evaluated 3-dimensionally, and the teeth and nose must be considered in addition to other facial and skull features. You child may not be a candidate for a chin implant, NOT because of his age, but because it is not the correct procedure or correct timing for the procedure.
Since this is such a complex issue, you need to have a frank discussion with the plastic surgeon as to his reasoning. Get the answers and then go from there.
sek