R.A.
I honestly felt like you jumped the gun. I also think that if you continually keep your son in a bubble, and switch schools every time you feel threatened, or think he is going to do something bad, he will never learn to handle things on his own.
I'm all for cautioning and advising your child to steer clear of the " bad kids" at school. However, I find it extremely inappropriate to go through another child's privacy, such as his backpack. Without his permission. I really don't care if he had pot on him, it was extreme. I am sure that that really helps your son make friends at his school. Things like this spread rapidly from kid to kid. It will isolate him even further.
I also think drug testing him every week is ridiculous. How are you going to keep his trust, and have him trust you if all you are doing is accusing him of smoking pot, and testing him every week. Regardless of if he is doing it or not. Does he feel pressure?
Any child who is not comfortable with himself, or he does not know his own values,expectations, and is impulsive will have these kinds of things to deal with. Finding friends, can be very difficult. It is often the case that when new kids go to a new school, and if they aren't comfortable, they will gravitate towards the kids who take interest in them first.
Did you meet with this kids parents? Did you establish a relationship with them. Did you express concern of finding pot in their child's backpack. Did you explain to this kid that you are not comfortable with his choices, and that if he wanted to come over, he will not bring his pot with him?
I have worked with many teenagers and adults with addiction and mental health issues, so I know the case when things get out of hand, and the decisions they make contribute to their decline. However, cautioning, and being proactive is a lot different then prohibiting, and rejecting. Since you rejected this child, your son is now being secretive, and hiding his feelings and friendships with others.
In order to establish trust, you need to be open minded, as well as have an open and honest relationship with your child. You would make sure that this kids' parents were aware of what was going on, and that you don't appreciate having him smoke pot around your son. That if they wanted to hang out, he cannot smoke pot as it makes you and your son uncomfortable. It is always more important to be direct with your feelings to your son and to whomever is around him. Also, by not rejecting this kid, you are letting him know that you accept him, but do not agree with his choices. Letting them know directly, they will appreciate and respect you more.
Have you talked to your son about his feelings, and what his choices would be, and if he wanted to smoke pot?
I would let him know your feelings on the matter, tell him how scared you are about having him get into something like pot, and tell him that you trust him , but are having a hard time with him hanging out with others who make those kinds of choices. Ask him if he can be trusted. If he has smoked pot, let him know that you are scared of that too. Let him attempt to make his own choices, and feel that he has a say in his feelings and decisions.
Keep him focused on his strengths, and keep him interested in the things he enjoys. Let him learn to make choices, and learn from those choices. If he doesn't he will just know that every time something " bad" happens , you make it for him.