I'd certainly be concerned, too. Alas, I assume he's over 18, so parents can't "make" him get counseling or take his meds. I agree that you need to show your support--especially reaasuring him that if he does happen to fail, you'll still love him. It's hard to make that transition away from home with all the pressures of school at the same time.

Could you possibly take a trip to see him and take him out for a visit--such as to a restaurant-- and give him your assurance? Also, he needs to be encouraged to get any support the school offers--such as tutors, a counselor, etc.

Could you possibly talk to a counselor yourself and see what he/she suggests?

Also, perhaps he could benefit from living nearer home. What other schools might be available? Could he bring his GPA up by taking some courses this summer at a local community college? Could the powers-that-be be petitioned to cut him some slack due to depression? Surely such a letter as he wrote would justify that diagnosis, along with consultation with an official counselor.

Is it his grades only that has him in such a state or are other things going on? I hope he will be willing to open up to you. But I assure you he will not if you do come across as judgmental and unsupportive. (I don't sense that in you from your post.)

How about his professors? Could they be prevailed upon--talked to? I was a college prof for a number of years and I certainly hope I'd be helpful to a student in his condition....
Please keep us informed and keep this dialogue going if you so desire. I have two sons in college right now myself.