Woah.. I'm so amazed by how caring and loving you are to your friend, he's very lucky to have you !

Having been through mental health issues myself (including depression, self-harm and suicidal ideation), I'd say what helped me was how caring and supporting my best friend was to me. Seriously, he would always be ready to hang out and talk to me when I was at my lowest, and oohh god knows that I was a horrible person when I was depressed. But he kept supporting me and staying by my side and I'm very grateful towards him for that.

On the other hand, my others friends weren't as supportive as him sadly. Most of them just wouldn't understand and even judge me ("People like you are just weaklings" they said. Tysm guys ily)

Well, it may seem obvious, but what you can give your friend to help him is : love, care, comprehension and do NEVER judge.

It seems that you're already doing that though But remember that, even if you can provide some support, you can't heal him. That's the job of doctors.

Wishing the best to your friend and to you as well <3