Everyone has already given you excellent advice. I went to school 8 hours away and struggled with depression and anxiety and my grades suffered. I have always found talking to the professors the most helpful. Encourage him to go to therapy, but try not to be patronizing. My parents would often try to come and visit me (luckily, my dad traveled for work to where I was fairly frequently). Be as supportive and as understanding as you can. And definitely make sure he knows that you will love him no matter what happens, that, to you, he is NOT a failure. Tell him how important he is to you.

I think talking to a therapist for yourself might be helpful. They'll have more tips for you on how to interact with your son, while also reminding you to be gentle with yourself. This is not a reflection on you as a parent, so don't beat yourself up. Also, if he knows you're getting help, he might be more inclined to see someone himself.

Good luck