I'm in a similar situation so hope I can help a little.
Did your husband adopt your child as their own? I don’t mean legally, but I mean does he treat him as though he is his own child?
I have a husband who does, since my daughter was 18m. The “person who helped make her” (which is the phrasing I use, I do not use other dad, he’s not her dad, he has never done anything dad-like), is known about and spoken of when she asks questions or when he comes up, such as where we used to live and who with and why we moved.
Your husband has feelings too and if he's been providing for your child emotionally and financially then he does deserve consideration. He may feel it’s a slap in the face to call someone else dad who has never done anything dad-like.
At the same time it's very important to realise that children don’t just erase a connection to a parent, even if they've never met them. I know this from many experiences of friends, myself, and my child.
My daughter called this person ‘my other dad’ and I asked why she referred to him that way and she explained ‘I he's a bad person (I chose to tell her that he put us both in immediate danger), but he's still my dad so I'm going to call him my dad’
And it was yet another example of a child having that intrinsic connection with a parent simply by virtue of biology. So that is real and should be appreciate as part of the child’s feelings.
Never speaking of him again is absurd, can't do that. It will cause more harm.
I really suggest sitting down with your husband, as the child’s parents, and talking over exactly how you want to handle the situation.
I did this with my husband, and ultimately it was a decision I had made prior to him even being my husband, but we have been together 5.5 years now and he is an equal parent to me and has equal input.
I find it interesting, even though my daughter misses nothing from not having her biological father, in that my husband provides anything she could possibly need financially and emotionally, she STILL feels some type of loss of this other parent. Saw it with my friends too. It's a type of rejection they seem to feel.
You can't ignore this, but you should sit down and decide how to approach it with your child.
Here's the approach I take:
He can't be around my daughter, he's not allowed due to the seriousness of what he did. But my daughter has every right to know all about him and ask about him and know him as well as she can.
I don’t envisage her wanting to meet him but I know she will still have that curiosity. She may ask to see a picture which I don’t actually have.
Then they will be curious about other family.
If you can get any professional help on this then do. I don't mean social services, I mean paying for a qualified psychologist to guide you.
You can't erase that man from your child’s life. And your husband also needs to accept that. Calling him ‘other dad’ is a choice you seem to have made, or perhaps you felt you had no other option? You can consider calling him something else.
Does your son call your husband dad?