Hi Katherine,
Definitely look into increasing Choline, which can be done via supplements or increase Choline-rich foods such as Egg Yolks. Also increasing Medium-chain triglycerides (MTC) would be a great idea, which can be done with Extra Virgin Coconut Oil, and even straight up MTC Oil.  Then you would probably best starting from the milder Nootropics like Alpha Brain, then working your way up to Piracetam and other ‘Racetams if desired. However this is obviously much more nuanced since he is 90 years old, currently on medications, and has some serious cognitive issues. For this reason we are very cautious to strongly recommend anything without the approval and guidance of his Doctor, and we would suggest this is your best course of action. You may need to find an informed and up to date Neurologist for this, but we are not experts in this area sorry. Thanks!