My brother who was the apple of everyones eye turned out very similar to your cousin, we just could not figure out where we went wrong. Everything you mentioned he was into, stealing, drugs, drinks, fights, arrested several times, cautioned etc etc.
We did everything we could, sent him to college (that didnt last long) got sacked for being constantly late, (his work was 5mins walk from home), off he was sent to Pakistan, got caught by the police for having cannibis on him while he was there (paid them off) anyway although he was in Pak for about 2 years he was still immature when he returned.
Not quite sure when it turned for him, but thank GOD he turned a leaf, started working, in fact started to do well at his job, got his driving licence, started driving (legally, once all his points had finished)
Hes also started doing namaz. I think someone mentioned before mothers duas are very powerful, and this is test, keep praying for him and continue to support him, he will InshaAllah come back.
You will all be in my prayers, this is a very tough and painful time for the whole family.