Congratulations for taking the first step in getting you son the help he needs!
This is a very difficult situation to be in. I think the best place to start is to ask your son. If in the past he has been open to talking to you about his life (which if he admitted to using drugs before he is) then he will most likely be open with you now. If after talking to him you feel like he is not being truthful with you you could look into different ways of helping him.
Below I have listed a few ideas you could use if needed after talking to him:
1. Random drug testing- if you are paying for his University if he refused to be tested (most likely because of guilt) then withdrawal your offer to pay for school.
2. Talk to his friends at school- if you check out who he is hanging out with and tell them you are concerned with his behavior they will share especially if they too are concerned.
3. Consider talking to his doctor if he sees one regularly or contact a therapist for local professional help.
I also advise not to overreact until you give him a chance to talk. When you are ready to discuss with him go in saying that you were concerned for him and wanted to be sure he didn't need help with an addiction problem. Try not to be too accusatory with him. You want him to open up not shut down so "be gentle" like we moms are so good at being.
I hope this helps you during this troubling time. If I can be of any other assistance please don't hesitate to ask more questions.
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