Hi M.a.l.a,
I'm sorry to hear your boyfriends having such a rough time, he's very lucky to have you. I can only suggest what i think may be causing his mood swings, (i'm not a phyciatrist, but i don't think his has looked at the whole picture) When a diabetic's bs control is not good, they can often have mood swings especially when bs is high and irratible when low. Your boyfriend needs to stabilise his bs control to see if this makes a difference to his moods, then look at which insulins he is on. Lantus has made many people feel basically like sh*te, and so if thats an insulin that he is on, he may find changing it to another helps. I would suggest he tries these things to see if they help lift his moods, which may then help with his depression.
Take care,
Suzi x