LMS1, if he is losing weight, as my daughter did, he is using a lot. They don't just slip up when it's heroin or meth. My daughter was always using a lot when she lost weight. I'm not telling you not to support him, but don't fool yourself. He did not slip up. You can slip up and smoke pot. But you don't slip up and use heroin or meth just once. And then stop again. Sadly, and I wish I hadn't, I have been there. Also, my daughter could function quite well on meth, cocaine, ADHD drugs and the other drugs of her choosing. She worked at Walmart part-time in high school then went to Cosmetology School where she aced it, even though we got calls from school telling us they were worried that she was using cocaine because some kids had gone to them with concerns. We didn't believe them because she was working, going to school, and mostly I didn't WANT to believe it. But her friends were sort of right...meth was her drug of choice, then downers so she could sleep.
I think a support group, where you can talk to other parents, would both make you feel better, give you coping skills and teach you what to look for in your son...and how to cut through the BS. I could not have gotten thru it without Al-Anon. That is available to you, as is The National Alliance for the Mentally Ill and other sulpport groups. Don't try to guess or do it alone.
If he comes home and you see any evidence of drug use, then you have some hard decisions to think about. No matter what, you can always offer your love and emotional support, but that won't stop his drug use. And, take my word on this as I was fooled so many times, you have no idea if his nice girlfriend uses drugs or not. Some very nice people use drugs. She would not want to be friends with a drug addict, even if he is in rehab, if drugs aren't ok with her. Birds of a feather and all that. Are his other friends who use drugs out of his life? That's a big clue as to whether or not he is seriously in recovery too. My daughter is a lovely person and wasn't even mean on drugs, except to herself. (She is even sweeter now that she is clean. I am very proud of her. IT CAN HAPPEN, but we must be realistic).