It sounds like your mentioning sui thoughts evokes fear in him. (We respond out of love or out of fear.)
It's possible he fears abandonment and experiences your times of feeling like exiting this world as a potential abandonment of him. He responds by questioning the relationship. (He doesn't understand why the relationship isn't enough to keep you from sui ideation, etc.)

If so, he's misunderstanding your illness, not the relationship.
He's questioning the relationship rather than questioning what occurs due to bipolar depression.

Just a thought.

I hope you both can talk more about this.