Hi Katherine,
Definitely look into increasing Choline, which can be done via supplements or increasing Choline-rich foods such as Egg Yolks. Also increasing Medium-chain triglycerides (MCT) 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 start from the milder Nootropics like Alpha Brain, then work 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!