@dederickve
You asked why he can act logically around others and around you his issues show up.
One idea I read about in a dementia book I am reading is called, 'cognitive masking'. This is when the person can't access the info that was stored in their brain, so they basically re-route and the brain goes elsewhere to retrieve the data they seek, so... doing so let's them mask their issue so they can function using info from a different part of their brain. This requires a lot of brain power and it's mentally exhausting, so since he feels safe with you, he can let his guard down and no longer have to do the heavy lifting of masking his issue.
I may not be explaining it too clearly, but one analogy is when Porky Pig stumbled over a word he could not get out of his mouth, he would substitute another word or phrase to communicate his thought.
Cognitive masking is done by intelligent people with memory issues, so they can hide or mask their deficits for awhile.
Big hugs to you. 🫂