What to tell your child why they won't see there step father anymore but there sibling will?

I have a 5 year old and a 3 yesr old the partner brought up my 5 year old since he was born and brought him up like his own we then had another child who is now 3 years but I have split up with my partner now and he has made it very clear that he only wants to see my 3 yesr old who is his and tells me he will have everything its breaking my heart now about my 5 yesr old and just do no hoe to go about things i feel like not mentioning it as he is young and will get confused and would forget eventually but he is going to be so confused and asking things when he still comes to pick up my younger child what do I say if he starts asking things ☹

What to tell your child why they won't see there step father anymore but there sibling will?
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