although actually left twice but always came back as my son needs a relationship with his dad
He can still have a relationship with his dad without you being in a relationship with him.
And if he doesn't maintain a relationship with his son because you break up with him, then he's not a decent enough dad to be bothered about tbh.
Your mum doesn't want the same path for you but you don't need to take the same path. She was an unhappy single mum who obviously felt unsupported.
You can be a happy, mentally healthy single mum after leaving an incredibly unhealthy relationship. You can be a single mum who hasn't been repeatedly disrespected, hurt and devastated by the same man.
THAT is how you break the cycle. It's not about whether parents are single / together. It's about whether or not individual parents are happy, stable and modelling healthy behaviour.
Neither of you are doing this as long as the relationship continues.
And I'm so sorry but if he's been doing this for so long, I would literally bet my home on the fact he has physically cheated on you at best a handful of times but most likely many times.
You need a plan to leave the relationship so you can prioritise your kids. And in the meantime double up on contraception as it would be unfair to bring another child into this dynamic.