> He might repeat the same question within 5 minutes-- this happens when he's really tired.This could be perfectly normal age related deterioration: https://en.wikipedia.org/wiki/Memory_and_aging
There are things you can do to reduce the effect, but it will affect most of us to a certain extent eventually if we live long enough. Heck, I show such symptoms when tired at 37, have done for years, and suspect everyone does too. Also, my late grandmother had problems with names (she'd sometimes run through a couple before getting the right one, even flipping genders part way through the list) but otherwise seemed to have a fairly complete and pretty well polished set of marbles until the end.
Of course if you have any concern you should broach the subject of consulting a relevant doctor. It might be nothing (well, nothing unusual) but it is worth being slightly paranoid with such matters in case it is something significant for which there are treatments to delay, mitigate, or possibly block, more serious ramifications.
One key seems to be remaining active, both mentally and physically. Staying mentally active is a no brainer here (sorry...), and recent studies seem to indicate that the old wisdom of staying active prolongs not just physical fitness but mental awareness and other abilities too (https://www.newscientist.com/article/dn28001-why-choosing-th...).