Hi, I can help today
I would suggest that you keep it simple. He is too young to cognitively understand so reply in an age appropriate way. I wouldn't ignore it . If you showed him a clip of his dad and told him that is who he is that is all he needs right now. If he asks why he isn't there answer simply he lives elsewhere, don't get into specifics then change the subject and reassure him that Mom is there to take care of him. At this age that is all he needs. Please accept so I get credit for my answer.