I like what adams said – to forget frumkeit for now.
Focus on his safety and “middos”. Teach him that even though he doesn’t want to live by torah rules, he still has to be a good person.
No drugs, no other illegal or dangerous activity, no dangerous people. And to have SOME structure to his day.
Otherwise he can do as he pleases.
Hopefully, this will teach him appropriate conduct, and perhaps he’ll return one day. At least he’ll turn out to be a decent person.
Again, easier said than done. Hatzlacha.