I think, for your son's sake, that he is fortunate that his boss is prepared to be an advocate for him and give him a second chance. Many wouldn't and in the current state of the economy, if he were sacked, he would probably find it difficult to get another job. He would then drift ever downwards, unable to pay his bills, possibly ending up turning to pretty theft to pay his rent, and then being evicted.
Firstly I think you need to get your son over to your house for a long and frank discussion. He will resent it, feeling that he's an adult and that he's entitled to his own life, but he must surely know that his boss lost his own daughter and has bitter personal experience of what drugs can do. I think he needs some straight talking to understand that his adult life is now probably at a crossroads, and he can go up, or down and out. Try if you can to educate yourself more on the drug scene. Your local library may have some informative books, and there's an Al Anon organisation which offers support to the families of those with addictions of various kinds. If there's one local to you, join in. You may be able to get some practical help and support in trying to influence your son to get off these substances.
I would also stop your younger son going over there at week-ends. How do you know that he's not being encouraged into a similar habit? For all you know, he may already be doing so. If he's not, try and persuade him to use his influence on his older brother. He may listen to him, as a mate, even if he won't listen to you.