Do you think it could possibly be depression? Certainly if he never gets outside, he (like most of us) is low on Vitamin D, which is a big mood thing ... easy fix, though. Also, an IQ of 183 is ridiculous. Here is what I have found with very smart people: they tend to think they can outsmart their physicality (my IQ is nowhere near 183, but I used to think the same). They can't. None of us can outsmart our humanity. Sufficient healthy food, sufficient exercise, sunshine every day, keeping busy in productive ways, doing for others: those are important components of happiness, and some of them (feeling needed, for example) naturally decline in old age, especially post-retirement. At least in Western civilization.
Grandpa and
Spideyman have it mostly figured out, sounds like.
It's true that the aches and pains of old age are not to be scoffed at ... but also, many of them are not inevitable. It's always worth checking with a proactive doctor (you know, the kind who believe in good diet and exercise and checking out what might be wrong) to see if the creaks can be uncreaked. Yeah, he might say "degenerative arthritis, not much we can do beyond pain pills, sorry" ... but he also might say "try these strengthening exercises." And so what if you need a pain pill? There are worse things.
Of course, getting a crabby old guy to accept all that is a challenge.