It's gonna be BIBLICAL
First off, you need to document and quantify his mental status. Take him to a doctor, where they can perform a Mini-Mental Status Exam (MMSE), which is basically a small cognitive exam looking at different forms of cognition (Visuo-spatial, verbal, math, etc). It's basically a grade school level "IQ" test.
Doing this well help document what his baseline cognition is now, and repeated exams later on will allow you and the doctor to quantify how fast he is deteriorating.
Really, there isn't much that can be done…..the first line therapy nowadays is donepezil. In the last few days, microdosed lithium has been shown to be effective, but not sure if it has been approved yet.
Otherwise, try to get him to listen to music from his youth. This has been shown to be one thing that can help.
Lapses in short term memory are not necessarily signs of dementia and can happen with general aging. The real warning signs are when he forgets things which has known for most his life….like directions to his home or who people in his life are. For example, a woman forgetting a recipe which she has cooked her whole life. This is why it's important to perform the MMSE in a clinical setting to determine the extent and type of cognitive loss.