I look at this post, and just erase the ADHD part of things, and see that your son is 20 years old, and is making poor choices (this is something that I drill into my 9 year old. You have ADHD, but it is your job to make good choices).

Your son is an adult, and needs to learn to make his own choices without your help.

It's really unfortunate that he is making a poor choice, but it is obvious by his repeatedly making that choice, he is not going to change just because you want him to.

Don't get me wrong here, as a parent, I totally understand the unending desire to protect your child, but there comes a point when you have to let them move on and make their own choices, good or bad.