what to do?

This is something I must wrestle with.  My middle guy, J, seems as if he's "just" using or experimenting like many 20 year olds his age BUT I am still concerned about him.  He seems "fine", is happy, doesn't fly off of the handle, has normal sleep patterns, is nice to have around BUT.... I am concerned about his "activities".  He rarely brings his long time friends around but the other night my husband and I came home early and saw his legal aged friends sitting around drinking.  Again - nothing out of the ordinary - they were all of legal age, not obviously drunk.  J isn't legal but he's been up front about that and he knows how we feel about that.   This is what I'm concerned about but I decided I'm not going to do anything about it, just yet.  I found our refrigerator magnet mirror in the basement where he and T hang out to watch TV.  Okay - not sure what that was about but I know it wasn't to primp and comb hair.  There is a full size floor to floor one in the basement.  So I bring upstairs and put it back where it belongs on the frig.  Well, the other night when we walked in the friends and J got up and went outside on our deck.  Most of, if not all, smoke so I didn't think anything of it.  Maybe that was what they were doing then.....but at some point they may have used the mirror because it was placed on the kitchen counter by the back door.  I asked J about that he is denied he was the one who used it and blamed T.  T wasn't around to defend himself but I know it wasn't him.  So I let it go and didn't argue with him.  Instead - I take his cell while he's sleeping (he's a heavy sleeper) and saw some conversations I wasn't happy about.  One - he's selling his Aderall.  Someone (maybe his new gf) asked him for some pain medication.  Looks like he's selling.  Not sure if he's selling anything BUT the Aderall because, by the way the text message was worded, it was kind of vague. Strangely enough, when I went for a walk this morning, I ran into the mom (on her walk) and ended up walking together.  She confirmed my suspicions that J was selling Aderall because she found it and found out where her daughter got it, from J.  She said she was going to call me but her daughter said that J doesn't do that anymore.  And where did he get the Aderall?  I filled it.  He has been so responsible, he's an adult, he excelled in school, and I filled it for him so that he could become adjusted to the meds before school starts.   So -- I think I know what to do.  He will be asking me to fill the next prescription soon because of school and do you know what?  I will tell him that because of T I feel much more comfortable if distribute it to him when he is ready for one.  I know that that will not sit well with him so I will then say "I know what you've done with the last prescription and you can lie all you want but --- I know".  This will not prevent him from selling anything else, if he is, but I certainly do not want to a part of this illegal and unsafe behavior.   Never a dull moment.....