Hi thanks for your letter
It's really hard for children and young people when parents split up. It's good to read that you do spend time with them both.
Quite often parents will find new partners which can sometimes be really difficult, not just for you but also for the the new boy/girlfriend. As you say in your letter, mum's partner is saying things about your dad that he should not be saying.
Your parents splitting up has nothing to do with him. You love your dad, mum and dad split up for their reasons. Nothing to do with you and certainly nothing to do with this new man in mum's life.
It sounds as though mum is trying to tell her boyfriend that he is out of order saying what he does but not doing very well. I think you are right when you say you shouldn't say anything to dad as it would upset him.
However both you and mum are being upset by her boyfriend and it is up to mum to take some action.
Possibly you could say again to mum how hurtful and unhappy her boyfriend is making you as you love dad.This may give mum some courage to make some decisions about this relationship or even tell your dad for support with this problem. I am sure your dad would not want you or mum to be upset like this.
Take care
Sam