How to deal with a bipolar boss?

So I work for a small business. And I only deal with the owner. The issue is he is very bipolar. One day he can be very nice and helpful. Other times he can be very very rude. And won’t answer me when I have a question. How do I deal with it? It drives me insane to the point I just want to walk out and quit. I cant though until I find a new job. I’ve been applying but no luck yet. So for now I’m just asking for advice on how to deal with this and to keep my stress levels at bay.

*there is no Hr. It’s a small business . He is the only one