RainMinusBow "But even if I tell him he simply won't believe me. He will go back to his dad and ex will say I am making it all up." To be honest I don't think it is your responsibility to get him to believe you. You need to be honest in age appropriate language with your children (as I am with mine, who have different but not insignificant issues).
So for example, my son is adopted. I need to be honest about birth parents (in a non-malicious way) and do my best to prepare him for the future. I am not responsible for whether or not be believes me as long as I am doing my best to be truthful in an appropriate way.
I think it is the same for you.
Even if you did leave your ex to have another baby with another man, because you chose to. That is your choice. Kids cannot make us stay in unhappy relationships, and they should not be encouraged to believe families should stay together where the couple are unhappy, it's not healthy.
"Son was also really rude to my OH - calling him an "arse-wiper" OK, kids are rude to adults, I know. I would simply address this by requesting he stop and disengaging while he is being rude.
I would also add that when we are rude about adults who are about us and for us, we look small. Say 'I hate to see you looking silly when [name] is always respectful to you. "(assuming it is true).
In the long run he may see his behavior is not appropriate.
I might also add "I would assume your dad doesn't really want you to be rude about people who are caring for you." And if he does, that shows his dad in bad light, so I might say "I think it is a shame your dad is encouraging you to be rude to [name], it doesn't show your dad in a good light either."
"...because he works in adult social care and saying he's a thick idiot because he only earns £20k a year and his dad has earned £40k in just one day today." I would not engage in this discussion, it's pointless. your son is being rude but then he is in a terrible position. He is having to prop up a lie, that his dad is the good guy.
Change the subject, disengage. you require minimum good behaviour. Get that however you can and don't allow your son to de-rail you. You are the adult. Your ex is acting like a twat, but you are the bigger person! Be that bigger person and don't allow your son to derail life.