No matter what world you think he is in. I'm sure he knows that he is being ignored. Engage him in conversation.
My grandfather has this and by all means I do not ignore him...I carry on a conversation and if he is in his world then for that time I live in his world. It's amazing what I learn from a 92 year old. He has discussed episodes while he was in WWII or working in the field. Living history.
I do not discourage him from his world / time as it only confuses him.
Actually when he is this way...we interact and eventually he comes back to the present.
Relatives should interact with him...it is sad to ignore him like he means nothing. Sad
Zig