Hello:
As a LBD patient I can understand some of what you are describing. You as the caregiver are there everyday and notice the changes - maybe subtle and others more in your face. Some seem to show up out of nowhere and others gradually do their thing. You know the whole scoop.

How long ago was your husband diagnosed?

If I may ask - is your husband aware of his condition? Is he able to communicate to his son about it? I ask because sometimes when a person refuses to accept a diagnosis or other changes in life - if the person that is afflicted acknowledges it then they might start to accept it.

Are there other children that are dealing with this situation?

I go through the fluctuations and like your son in law but my oldest son doesnt see that. Since I told him a few years back about the diagnosis I could tell he didn’t want to believe it. I see him a few times a week along with my grandsons. That always brings out the best in me. So when I see him I’m happy and a little more social and with it. As you mentioned - you are there 24/7. You know about the swings, and the highs and lows. He only sees the good days.
When my son sees that, it builds on his non acceptance.
Now -over time - he does acknowledge some changes. Your challenge is that his son doesnt see him often enough to experience the other side of things.

Do you or the family have chats on a platform like zoom? That might be a way for them to see the not so good times.

Do you think a video of when your husband is feeling the nasty effects from Lewy would help?

You have an extra challenge with your son in law being in the medical field. He is correct - unfortunately - in his statement about a diagnosis can only be 100% known at autopsy.
Has he ever acknowledged that there might be an issue but just not LBD?

You mentioned several docs across different disciplines working together to support the diagnosis. I’m sure your husband and yourself have endured neuropsych testing.

What other tests have they used to determine the LBD diagnosis?

Do you feel it might be possible to have his son talk with his docs to hear what they have to say and then he can ask questions? Maybe coming straight from another medical professional it would help?

This is a disease that not only affects the person with it - it is a whole family thing.

Sometimes family counseling is in order.
Have you considered that possibility?

Please keep posting and you will find many caring and compassionate people here that have it in their hearts to help.

You are not alone - use Mayo Connect as one outlet to share concerns and yes - joys!!!

Peace
Larry H.