Deuce, I did exactly what you are contemplating doing. I "adopted" my son's friend after his mother died. He was 17 at the time. I knew he smoked weed (a lot), but had no idea that he was into other things as well. I loved him, but was shocked by his lack of respect for me and my property, his defiance, his willingness to just let me pay for everything for him while he blew his money on stupid things and drugs, his COMPLETE lack of appreciation for everything I was doing for him. Things got so bad after a year that I threw him out. He lived with a friend for a few months and alienated the friend's parents within that time. I let him come back to stay with me when those people had had enough, with all sorts of conditions. He accepted them all, then promptly broke all the rules. One of the big ones was no drug use in the house, but I was constantly finding evidence that this rule was being broken daily. When I would call him on it, he was indignant and incensed that I would invade his privacy, and he demanded that I stay out of "his" room.
Last Fall, I discovered that he was a heroin addict. I knew that he had been snorting Vicodin (I thought - I regularly found evidence in his room), and I knew that he liked pain pills, but I didn't realize he couldn't stop even if he wanted to and didn't know heroin was on the scene. Caught him trying to shoot up in my driveway in October. Muscled him into an outpatient rehab program, followed by an inpatient program when the first didn't work and after he was arrested for stealing to support his habit. Inpatient didn't work either, and I threw him out of my house again. Ultimately, he was killed because he was in the wrong place at the wrong time due to his drug use.
What a roller coaster ride, with such heartbreak throughout and especially at the end. Through it all, I learned what everyone on this board keeps saying - you can't "help" them if they don't want to be helped. Giving him a soft place to land just gives him the opportunity to continue his use. His mom threw him out for a reason - what makes you think it will be any better with you? Because you are essentially a stranger, because you are being so kind in offering your home to him? So therefore you expect that he will show you and your home respect and appreciation? Don't bet on it. If he is an addict, he will just use what you are offering, not appreciate it and will not show you respect. You will be amazed at the lack of respect; I certainly was. If he's working on recovery, that's a different story, but if he is in active addiction, he will behave like an addict. It doesn't matter who you are or what you are offering.