Thank you so much for your responses, they really have given me a lot to think about.
I really hesitated to use the term "secondary" as I dislike the label, I was struggling to find more appropriate words.
I do agree that I need to approach a conversation with him in terms of what support he is able to provide, rather than what he's not doing.
I can appreciate the differences in both my husband and boyfriend, their differences are what makes each relationship special to me. Feeling like I'm walking on eggshells around my boyfriend is an adjustment. I'm very used to being able to speak my mind freely and I find that my timing and approach as well as his energy levels and emotional state can be a delicate balance.
I really wanted the perspective of poly folks in similar situations as much of the literature available on depression is specifically monogamous. I've been reading books and browsing through forums about depression and they are a bit overwhelming. The depression forums that I've found were, quite honestly, negative and full of hopelessness. So many people aren't able to stay together through mental illness.
I would agree that more of a support network for him would really help a lot. I've encouraged him to reach out to other friends who have experienced depression and are happy to be there for him. He hasn't taken them up on it as of yet. I also have a real problem with medicating a mental illness without any ongoing therapy. He just goes in to a doctor every 6 months to adjust his dosage. Medication does help, I don't disagree with him taking it. I believe that he needs help to better understand how his depression affects him. He wants to go through life with things as calm as possible, but when shit happens, he doesn't seem able to cope very well.
I don't know what the future holds, but I was looking for a bit of hope. I believe your answers have provided me with that and I truly appreciate it.
"There are worse crimes than burning books. One of them is not reading them." - Ray Bradbury