Hi chickpea, welcome to the forum
Wow! My heart goes out to you, I can imagine how hard this must be on you.
I'm a depression sufferer and a few other things. I put my wife through hell, much like your boyfriend is doing. There realy isn't much else you can do other then try to support him and continue to try and get him to seek help. Unfortunatly, until he admits or recognizes that there is a problem, he will fight you.
Perhaps you could try providing him with a list of the symptoms of depression, since I don't think he will look for this himself.
Keep us posted and good luck.