Colleen, I have been where you are now and it really really hurts, like a pit in my stomach. I had to mourn the dream of my daughter going to college and making a successful life also. Every time I saw or heard about one of her friends that was doing well in college or getting a good job and moving on with their lives I relived the pain. It took me years to stop wishing for what was not going to happen.
Of course he is depressed. When my daughter was using pot all the time she slept a great deal. She had no motivation and seemed very depressed. I did have to make her leave out house after it was apparent nothing else was working. There were times when she texted us and said she was going to kill herself (she was a drama queen and this was not unusual for her). We had to tell her we loved her and hoped she didn't do that but that she was killing herself with drugs and alcohol.
I would never tell you what to do, we each have to do what we feel comfortable with, but I don;t think your son is going to suddenly decide to stop using pot and turn his life around without some significant event happening. It may come to giving him an alternative of getting help or finding another place to live. That's the only thing that help my daughter.
Every time I read one of these stories here it makes me angrier that so many people think pot is harmless. It isn't to many of our kids.