I'm sorry your son is having such a terrible time with depression.
But . . .your son has got to know that CF is NOT the "die young, life sentence" it used to be. I know plenty of CFers well into their 40s, 50s and 60s. Struggle, sure we do, but the older CFers I know generally have a great frame of mind and have learned to deal with the CF challenges. I guess it comes with acceptance of CF and realizing it's really our only option. A BIG realization expected of a teen, I understand. But doable if he realizes that he can live into middle age and beyond.
I'll try to be of help with what has worked for me. I'm 53, and have had my share of depression on and off over the years. Unfortunatly, depression generally comes as a package with CF. I have known a lot of CFers, and everyone I've ever met, deals with it at one time or another. Teenage years are a tough time for everyone, let alone dealing with the realities and routines of CF.
I have done the counseling route several times and the drugs route, which you did not mention. For me, the drugs got me through a particular tough time until I could get back on my feet again. I am a firm believer in setting goals, planning ahead, and looking forward to events, happenings and better days. Staying involved in things I LOVE to do and exercising even if it is a nice walk are both great mind cleansers.
Since you say he is healthy, he should try to focus on that incredible part of his life. Being preventive and proactive in his treatments, meds, eating right and exercise will keep him in a healthy physical state much longer. Does wonders for the head as well. Working at staying healthy is easier than playing catch up, trying to recover from illness. I know it feels impossible to get him motivated to so something when feeling down. Maybe you, Mom, can get involved and take him bowling or whatever. Encourage scheduling things to do with his friends, keep him busy with fun things.
Probably THE most important thing I have done was make a bucket list, or wish list. 9 years ago I was on death's door, had my left lung removed and gradually recovered. It made me realize how short life really was . . . I still had much I wanted to accomplish. As I laid in my hospital bed, I made my list, things I wanted to do the most in my life. As soon as I did this, I started making plans how I could make those wishes come true.
And you know what? I have done many of them. I've lived more in the last 9 years than all my others put together. I constantly add to my list and look forward to making my dreams come to fruition. I know it sounds simplistic, but it works for me.
OK enough rambling.
Good luck, I wish you and your son all the best.