If your son is serious about preparing for Chemistry Olympiad, then yes he needs to get the knowledge of “AP Chemistry” this year despite not being allowed to take the course at his school yet. How he does that is up to your family (online, tutors, asking for an exception at school…). Most successful USNCO participants do not wait until senior year to take AP Chemistry.
That said, the actual AP exam this year is not necessary, unless you think it will motivate your son to really learn the stuff.
It would be ideal for your son to be working with the AP Chemistry teacher at his school, and further ideal if this teacher shares your idea that your son is very advanced in chemistry. That person can also advise on the best path forward.