Hi Mayham,
Right now, you're wrapped up in what's going on with your son, which I completely understand. My 16-year-old is a pot smoker, too, and so far, I haven't been able to stop him. I have stopped trying to control him, because it simply didn't work. Also, it was making me angry, miserable, and crazy.
Buckle up for your journey. It's a long, slow one. Counseling for him is a good first step. There is also family therapy, if your husband will agree. If he won't, I would suggest Al-Anon for you.
With counseling, your son may decide to back away from pot. If he does not, Al-Anon will help you understand that--regardless of HIS choices--you can choose to get help for yourself.
Tuck this in the back of your mind: You didn't cause it; you can't control it; and you can't cure it. Not even with love. If love cured these types of problems, few of us would be here.
There is another way. Al-Anon will show you what it is. I've only been going to meetings for a short while, and they are extremely helpful.
http://www.al-anon.org/meetings/meeting.html
I know it's scary, but there is hope. You can come out the other side of this.