In my opinion from what I read, you have a 20 year old son. Males do not reach full brain development until 25 or so. I use to be one of the nicest most respectful kids ever growing up. By the time I hit 16 my mother and I went to battle against each other for years.
I feel as though weed is no big deal. However taking into consideration of the purpose of this site, substance abuse is never a healthy outlett regardless of the drug of choice.
To this day I smoke weed everyday (hardly drink at all and when I do I dont have more than 2 drinks in a night as it just makes me sleepy). Ive successfully graduated college smoking weed everyday all throughout my educational career. Ive always had a hard work ethic and cobtinue to work my butt off everyday. Even so, my last job was eliminated andbit took me months to land a new one.
Back to the school part real quick, your son has accelerated to finish school as fast as he has so far. Going to work and veing a full time student is next to impossible. So long as his behavior foesnt get out of control, and he staus on track to finishing school dobt force any other changes upon him till then.