No, but you have my heartfelt sympathy. I am not an expert, but the valium might actually be helping him. Don't you have to get it on prescription? In which case, he must have seen a doctor, which means that someone is monitoring him, and he has had the sense to seek help.
Awful behaviour + stealing = (most likely) drugs + some level of unhappiness, I think. According to my three nephews, all aged 19 - 21, recreational drugs are par for the course at this age. And, it has to be said, the behaviour of many kids in their late teens can be horrible. How long has it been going on for? This is a really difficult age for almost all the young men I know. Long holidays from school, college or uni, no money, no job ...
But stealing? Is it to get money for drugs? How much/often/when did he start stealing? What did he say when challenged about it?
Does he have a job or is he at college? Do you know any of his friends? Can you get them on side?
Above all, where is he living if not with you?
I'm sorry not to be able to help. Somehow you have to talk to him. Maybe you could see a counsellor?
Good luck.