Rkm, I just want to say you're doing the right thing by showing him the consequences of the law and how his actions are going to put him face to face with the scary U.S. justice system if he does not tread lightly and use some common sense. I think it'll be the wake up call he might need to realize that cannabis is illegal for now, and if he's going to partake, he's gotta fly under the radar like the rest of us.
As for what may happen, it's just a PI and his first offense, so he could easily get off with diversion (it's a plea of guilt, but it shows up on records that the case was thrown out), and maybe at the worst a strict probation. In that case, stay on him and make sure he stays clean (or at least has a job for some detox) so as to pass the piss tests.
I've been through a couple times, and my own father was great in that he told me he wasn't going to spend a lot of money on a lawyer because I had to see that my actions with marijuana could have deep consequences because of the corrupt "drug" systems in our country. Then he rolled the greatest spliff I've ever seen, and taught me to treat cannabis with respect, because it's sort of like the Universe's gift to us to help us take time out of our day to relax and open up the senses. It was about the most spiritual thing that atheist ever said to me.
I would never think to tell you how to raise your kids, that's the biggest douche thing to do, but I will never forget the day when my old man decided it was time he stop taking a passenger's seat to my ganja-infused life, and show me that it will all be okay someday, but for now I just had to be patient and smoke where it's safe.