I have been through this..My Ds hasn't seen his Dad since 3 now 8... We have been through lots of stages.
I have always been honest...Not slagged his Dad off but not made excuses for him either ( I have not wanted him to feel anger or hero worship) I have always tried to be age appropriate and be able to build on what I have told him.
At first I told him it was something his Dad had to arrange , We ended up in a refuge. I told him that I decided that we would be happy living away from Daddy and I made the right decision.
Now he is 8 , he does know his Dad had anger problems, I also told him that if his Dad wanted to see him now he would not have the right to just see him. He would have to go through a process to make sure that it was the right thing for him ( refrained from saying over my dead body)
I did repeatedly tell him it wasn't his fault.
It was like a revolving door situation. It seemed to be generally solved now since a teacher spoke to him after an issue came up due to a subject at school. Her telling him it was nothing he had done as he was a baby seemed to help.
I have also told him he has another DC who he doesn't see.
I from my personal experience think answer what they ask.
Charly bear ..If he doesn't want to talk about it leave him, he will or won't in his own time