Yes but the truth is not point scoring op.
I would be careful framing nasty behaviour as as a 'mistake'. Because it's not. Your son will already be looking for every excuse he can for his dad's behaviour.
A mistake would he if he got angry or he upset you or his son once and then was horrified with himself, appologised and never repeated the behaviour. But it looks like he repeatedly hurts your child by making him feel like he isn't good enough, and then gets angry at you when he comes to you for help. That's not a mistake. It's deliberate bullying. If you keep telling your son that cruel behaviour is a mistake then he will think he has to accept and forgive it.
You say he has low self esteem. This is probably the reason. His dad treats he brother well and yet frequently, reduces him to tears. He thinks 'maybe it's my fault, maybe I do something wrong to provoke him. Maybe I'm not good enough'. If other people tell him 'oh your dad makes a mistake but he regrets it' ...then what he hears is 'so it must be me overreacting. I should just forgive and forget. Put up and shut up. This is normal'
What he needs to hear is the difference between acceptable and not acceptable behaviour. And that it is not OK. And that he doesn't have to accept it or excuse it ir forgive it. And that it is not his fault.
Frame it using examples of healthy behaviour vs toxic behaviour in life for example if you dont want to use specific things his dad has done.
But be aware that promoting the mindset that nastiness is just a momentary mistake from people who well, have form for nastiness, is potentially setting the lad up for a lifetime of tolerating abuse and choosing abusers because his boundaries are not where they should be.