I don't think your father is getting anything from the interaction and your son is too young to be getting anything "positive" from being around someone with dementia. I wouldn't "bother" with putting yourself through this; I don't see a big enough payoff for anyone. If he can't remember his grandfather before he had dementia, what is the point of remembering him in this way? Obviously your son has an issue with seeing his grandfather, I know my father's last days; when I was in my 40's were not pleasant and I'm "stuck" with them in my head even though I had a lifetime of good memories! How much more a young child with no offsetting memories of old or sick people?