I have suffered from depression, and can identify with your bf.
Like other contributors, I wonder how much of his wanting to be in a relationship with a poly, and to be the secondary in that, is a matter of him feeling he has not got a lot to give.
However, I would not suggest making this point in the form that you want to get as much as you give: if he feels he has little to give, he might interpret what you are saying as a request to cut down what you are giving to match his perception of what he does give.
Instead, I recommend finding ways he can support you: whatever they are: emotional or practical or whatever. (Real ways, don't pretend, of course). Emphasise them along the lines of 'if you can do X it will help me a lot'.