I'm unclear on whether your ex has actually disappeared, or whether he is near by and just turns up occasionally without your knowledge?
When did your DS last see his dad and does he ask about him/want to have anything to do with him?
I think honesty is the way to go, but glossing over things for now, at least until he's older. e.g., if his dad is nearby but doesn't get in touch - Dad can't come here because he treats mum badly. He can call and arrange to see you, but he hasn't. I don't know why.
If his dad has disappeared then maybe something like - Dad treated mum badly and got in trouble and I haven't heard from him since. I don't know where he is, but he has my number/email address and can get in touch. I don't know why he hasn't. I'm sorry.
And, of course, make sure he knows he can rely on you.
If the bit about him being able to get in touch isn't true, don't say it. If your ex isn't allowed to get in touch then say he treated you badly and now isn't allowed to get in touch.
I wouldn't, at least not at this age, tell him how your ex treated you badly. Just say it was serious enough that he can't see you anymore.
I think there used to be advice to tell kids that their absent fathers still loved them, but that that has fallen out of favour because as kids get older and realise it's a crock they feel it as a form of gaslighting. So I would avoid that. It might make him sad to realise his dad just hasn't bothered, but if you provide him with love and constancy, that will meet his needs and he'll bounce back well.
They key is to tell him the truth at a high level keeping to basic facts and without making it dramatic or letting your emotions drive the narrative. Let him feel sad/angry/etc. and offer him your love and support to help him over that.