According to the Mayo clinic, dementia-like symptoms can have numerous causes including some medications, thyroid problems, sodium or calcium being off, deficiencies in D, B complex, and C vitamins, not drinking enough water to stay hydrated, and low blood sugar - hypoglycemia. (My dad had hypoglycemia periodically and got very fuzzy, forgetful, and cranky. The treatment was some food with protein and complex carbs. Worked in a couple hours.)

If he is eating snacks instead of balanced meals, not drinking enough, and not sleeping well that could be the problem right there. The others could be checked by a review of his meds and a couple cheap blood tests. I'd bug his doctor about the calcium/sodium balance, thyroid, vitamin absorption, and blood sugar issues and if any of his meds could be causing the problem.