Wow, I grew up in a similar environment and went thru somewhat the same as your son.
I said earlier, I think that you have got to accept that he will not be frum for the time being and maybe never. At some point in the future, he may find he is missing Yiddishkeit and he can make his way back. I think your agony over this, is your problem and is possibly getting in the way of the next phase of his life which is getting into pre and adulthood, via either the Army, or becoming a sort of Yeshiva Bochur, University, or job training, marriage.
What I get here is your feelings are getting in the way of helping him.
I believe you should tell him that he doesn’t have to be frum. I see this as a core issue. I am speaking from personal exp. I was your son once.