Welcome...I'm so sorry for the pain and fear you are experiencing, but very glad you are reaching out. I think speaking with him is a good suggestion, but be prepared for him denying the pill is his or denying use. I don't know if he has an addiction, but from what you are explaining, it sure sounds as if he is headed down a path that could lead to addiction...opiates are awfully powerful and abuse can alter the brain and make quitting so difficult. While it is hard, I know, because he is so young, I think it is also a good thing because you can have some say in what happens. Please seek any assistance you can, perhaps speak to a school counselor, family doctor or a counselor from the program he attended.
I know you may feel guilty, but you did not cause this nor can you cure it. You can, however face the situation head on and not worry about hurting his feelings or making him angry. He is still a minor, so I would urge you to do all you can to try to address the situation now before it progresses. I also found face to face Naranon meetings helped me a lot to deal with how drugs and my child's addiction impacted me. Mom to mom hugs.