I think this is quite normal.
I have two sons, coming up 4 and 7 and when we are all at home together they show complete indifference towards my husband.
They want me all the time, they only want to play with me, they only want to sit with me, whenever they want something it’s me they ask for it, they only want to cuddle me, it’s only me they want to put them to bed and read to them etc and in general they seem to show a real preference for me.
However, when I’m not there they have the best time with their dad!!!! They all have so much fun together, they go on trips out together, they say how fun and exciting their dad is and they all have a great bond with each other.
But then when I reappear it’s like dad doesn’t exist again.
Thankfully my husband doesn’t take it to heart and we don’t let it bother us. Children can be complex and fickle.
Has he said anything about his dad that concerns you? As in why he is acting the way he does? Is he aware of how dismissive he may be appearing towards his dad?