Well, we're not experts, so first thing I'd suggest is seeing if you can talk to him about visiting his doctor for a referral to a mental health expert.
Hi and welcome :)
Do his signs of depression affect his work or his relationship with friends? I know you mentioned you take the brunt of it, but does he still socialize with friends as usual, get along with co-workers etc? This could help. If he refuses to seek help, you could always speak with your Dr. about how to talk to talk to your DH. I hope you find some answers soon. Keep us posted!