Genetic predisposition and chemical and hormonal imbalances can cause depression. However, given the misfortunes that can befall us during our lives and the continual bombardment of bad news that assails us every day, those of us who do not have the misfortune of being disposed to the condition may face a daily struggle to not become depresssed.
If you had not mentioned that your dh is depressed I suspect that you would have been told in no uncertain terms that he is behaving like a Class A twunt, and you must not allow him to use his depressive state as an excuse to beat (metaphorically speaking) on you.
If he's going round in circles reliving negative past events, he needs to stop. CBT can help him with this, but replacing negative thoughts with positive ones is something that he can start working on now instead of waiting for a counseller to show him the way.
It would seem, from what you've said, that his experiences during his last employment possibly exascerbated any pre-existing problem he may have had with low self-esteem and poor self-image.
It's probable that, with hindsight, he could have handled the situation better perhaps with the help of the company's management/HR. In any event, he needs to take any lessons learned forward into future employment and seek help immediately if he has reason to believe that he is being bullied in his workplace.
What field of employment was he engaged in? Does he belong to a union? Perhaps he could use the net to ascertain if he has grounds to bring a case for constructive dismissal - it will be better for him to direct his anger and frustration into pursuing this matter rather than stewing over it and venting at you.
From what you've said, he doesn't sound paranoid in the classic sense. It seems to me to more as if he feels powerless to think positively and bring about positive events in his life (even though, as you've observed, he has so many positives under his nose) and I suspect this may be directly linked to his recent work experience. Pease mention this to any medics you encounter as he may be suffering from a form of post-traumatic stress syndrome.
On a practical note, make sure that he eats a nutritious diet and spends time outside of the house each day. Get him Vitamin B complex and zinc supplements - cheap as chips on eBay - and consider buying St John's Wort tablets, but he should not take this herb if he is prescribed anti-depressants.
I hope that you get a good outcome from your local A&E unit.