I don't have much advice in terms of your relationship but I can speak a bit about diagnosis. If someone doesn't want to get diagnosed they won't do it and that's speaking from my own experience. A friend pleaded with me senior year of college to go see a doctor but I didn't want there to be anything wrong with me. So it took me two years and consistent struggling to finally go see a doctor. If I could go back in time and get diagnosed earlier, I would but that's why hindsight is 20/20. So heads up, unless he decides it for himself, there probably will be a lot of push back on seeing a doctor.

Speaking to your relationship, I'm just going to reiterate what you wrote yourself, "It's all made me so horrendously depressed" -- that usually means something needs to change. In this case, I'd suggest moving out so you can look at the relationship without the stress of living together. Then finally figuring out where you see this relationship going, not just a year down the line but maybe five so you can figure out if living together in the future will even be plausible.