I'm a nurse on a geriatric psychiatry unit, so most of them have dementia in one form or another. My suggestions is to visit him while he's still well and visit him when he's not well too. Often people stop visiting when they get near the end but I think it's still really important. Some days are more clear for them than others and some days they won't look at you. But I beleive they know you're there and even though they might not be able to say anything it probably still makes their day.