I did what others weren't willing to do:

I helped him.

Instead of waiting him burn. I stepped in and really helped him.

At first he was afraid of some backstabbing from me.

Then once we trust we cleared all problems, upgrade whole structure and finished a year long project.

We did discuss, argue but once we cool off. We did get out to drink coffee as good friends then get back to work.

Tl Dr -> Help him or get out.

Don't be passive.

Ps. My old boss had lost his mother and was not coping well. Inviting him to lunch and coffee breaks helped a long way.