If he truly has depression then he won't be himself and even if he does seek treatment he won't begin to feel like himself for a long, long time.
Don't be surprised if you end up with some form of reactionary depression yourself too. So be alert for the signs and get help if you need it.
If he won't seek help then there's nothing you can do. Same way you can't make an alcoholic give up the drink, or a smoker give up the fags, or get a fat person lose weight. It's up to the individual to take the first step. But that's their choice, not yours, so don't let their problem become yours, because it's not. It's theirs.
If he does have depression, and unless you've been there yourself it is so hard to understand, he will need time and space. He'll ask for your help if he needs it. So let him know you'll be there for him down the track if he needs you, but for now the best thing you can do for yourself, your health and your sanity is to walk away and leave him to it. The old saying if you love something set it free, if it comes back it's yours. If it doesn't then it never was comes to mind.
You need to concentrate on you. It's hard when every thought in your head is about him, but trust me, in time, it does get easier to do.
So many of us here have plonked ourselves down and curled up in a ball and howled and howled and howled thinking our life was over. But hey, have a look at us, we're still here. I'm not saying any of our lives are easy, some days I'd have to say mine is f ing shi t, but I now know to give into the carp days and hang on in there coz tomorrow will be better and who knows what the future holds?
I once read some wise words and from rough memory they went something like this. When we're on a train and the journey we'd planned is interrupted by the train stopping does everybody suddenly just get off the train and walk away and not complete your journey?
No, we have patience and we wait. I'ts not what we'd planned and it's a damned nuisance, but you sit there knowing that eventually the train will start again and in time, maybe it'll take longer than you'd expected, but in time, you'll reach your destination.
This may not be the journey you were expecting, but have some patience and give yourself some time and trust me, before you know it you will be on your way once again to your destination.