I read your post, and it made me just want to add a few things that I have experienced, cf related medical issues, that can contribute to depression. If it does not help you, perhaps someone else who reads it may benefit so here goes:
1. I know it seems unrelated but if I am suddenly battle to cope emotionally, then there is usually some infection brewing. That is my signal that I should pay attention on my next clinic visit to see if my FEV1 has been slipping and if some action is required. Sometimes it takes someone else to point out to me that I have been in a bad mood before I realise this. Even if my body feels fine, I know there is a good chance it's not and I am just not noticing it because the decline is gradual. Because I have experienced again and again when the infection is under control, I can cope with life again. Depression is not always 'in the head' and can be 'in the body' also.
2. What is his sleep quality like and does he use O2? I suffered from depression for which the drs wanted to prescribe anti-depressants. But it was finally discovered that I was not sleeping well due to dropping oxygen sats at night, so I was functioning on low quality sleep for a long time even though I slept solidly 10 or 11 hours every night. Aside from being constantly tired, it also made me depressed. When I started sleeping with night-O2 my depression practically vanished because now my brain gets to rest and recharge like it should. Yes I still get the occasional down day, but mostly I feel like a new person. The doctors can do a sleep study and look at the sleep phases and O2 levels, to determine if this is a cause. Sleep O2 sats and awake sats can vary so don't assume if his lung function is good and awake sats are good that he is sleeping fine. I wasn't. (if the doctors are not open to this idea, ask them to do the test to give you the peace of mind that this is NOT a cause so that you don't treat the wrong problem)
3. Exercise: moderate exercise lifts my mood, so try having him to join some sort of structured exercise - perhaps something you can both do for motivation.
4. Diet: this was mentioned already but I will just add that caffeine is the big one I need to avoid, followed by sugar. Initially it may lift your mood but the energy slump you get after a caffeine/sugar rush can cause moodswings. So aim to include more water, fruit and veggies and unrefined food...you know, all that healthy stuff we hear on Oprah. It does help in the long run.
All the best
Lene