Music often has an amazing ability to transcend the effects of age.
I saw Dave Brubeck in a music hall in Chicago something like 10 years ago. He was in his late 80's and I watched with dismay as the sax player supported/led Dave Brubeck (hunched-over and shuffling his feet) to the piano bench. He was obviously riddled with arthritis!
I thought:
"This is going to suck. He is old and feeble"...a big enough name to draw a crowd and make money, but I was not going to get the fantastic music experience I had anticipated tonight.
He sat on the bench and there was a microphone over which I struggled to make out his slurred words announcing the first tune.
Then, the magic happened! The drummer started playing his 4 bar intro and I watched in amazement as Mr. Brubeck's back straightened and he sat upright! That f*cker played the concert as if he was once again 30 years old! It was one of the best performances I have experienced and certainly one I will never forget!
How in the heck does that work???
Sorry for the diversion, but getting back to your dad, keep track of his guitar and as he gets older and succumbs to the infirmities of age, hand him his guitar every now and then and ask "Why don't you play something for us" and see what happens!