Spouse doesn't speak coherently. How can I help him?

We need more details about the situation. If this is new it is a medical emergency! Ambulance now!
I once cared for a person with a brain tumour who's speech was hard to understand. I could understand him. I acted as a translator for him. People would ignore him and address me. I would say I'm the translator, please address Mr X directly.

Agree. If this is a change in his normal state, get medical attention straight away.

Is this new and sudden? He may have had a stroke. Call 911 immediately.