I think you may just need to accept that he won't ever understand, and may never accept your struggles.

My grandfather.....narcissistic and h-e-l-l....yeah, I'd say he was indeed to the point of having NPD (it was pretty bad), and had terrible social anxiety (retired at age 55 b/c he couldn't deal). Anyway, he never believed in anything psychiatric...I left college at age 19 b/c of my struggles (anxiety, etc) and his only words for me were "I hope you got that out of your system". Uhm, yeah...out of my system! (I have PTSD.) There was no explaining anything to him. I just had to accept it....and move on.