Oh babe I feel for you. I was very smug about my very antismoking DD until she started smoking at 17!
I told her that I'd take it up again which she is horrified about but it hasn't stopped her sadly.

I have no answers except that it SHOULD be very hard for him to get smokes as he is under 16 and shops can cop hefty fines for selling them to minors.
I smoked but havent for 10 years now and I know that nothing anybody said to me would've stopped me, all you can do is make it hard for him to get them.
Its no reflection on you as a mother so please dont feel like it is.