It's tricky with an 18 year old as any help a parent tries to give can be rejected because the young adult child is trying to separate from parents in this stage of development. So you might be seen as meddling or nagging and trying to control his life, so he might be apt to do just the opposite. The psychiatrist should be able to give him tips for how to remember to take his meds each day, such as a 7-day pillbox, an alarm on his cell phone that goes off at the same time each day as a reminder, etc. That is more helpful than just telling him he should take his meds each day, which is easier said than done for someone with ADHD who is prone to forgetfulness (I know that firsthand!). Your son may find it easier to accept advice from a doctor than a parent. Also, if the pdoc may have a nurse who can call weekly to chat with your son, check up on how he is doing with taking his medicine, and troubleshoot with him on ways to improve compliance.

I think if your son experiences significant improvement with the Vyvanse it can help motivate him to take it. He can see the results himself. I am on Vyvanse also, and it helps me so much to be productive, not forgetful, etc., but yet I often don't take it too. It is push-pull-push-pull. I want to be productive and more effective but I also hate needing a med to be that way. So I think I am not religious about taking it daily to show myself I can function without it. And I can function without it, but I function better with it. Push-pull. Not rational, I know, but there you have it. Maybe your son could commit at least to taking it on days when he feels it is really important to be at his best, such as very busy days with many responsibilities, test days, etc.