write or wrong-
two possibilities:
1) he/she reaches the realization that he/she has hit rock bottom-wherever he/she draws that line
2) he realizes that there are other hashkafot which are equally acceptable. He is clearly unhappy living the life you have set for him (mostly because he has been burned). He needs to realize that he can define a new identity for himself within the conforms of halacha. Also, it would alleviate a lot of hurt if you could research for yourself where actual halacha ends and social standards (chumrot and the like) begin.
I am so happy that things seem to be picking up! Many teenagers change their dress as a rebellion against the notion that one’s value can be based on their external appearance. It is possible I am wrong but I would venture to say that you should not be making as big a deal when he makes steps in the positive direction, just as you should not make such a fuss each time he crosses the line. He needs to know that you love him just as much on his ‘better days’ as on the days that he knows he is doing what he should not.