Now his parents say I’m being unfair and he can’t afford this much per month (CMS have added 20% admin fees) and won’t see our son until I cancel the application? They’ve said it that it’s my fault they my DS will now not have a relationship with them.
He can afford it.
It's not your fault he underpaid for so many years.
Stop discussing it with his parents.
In fact - stop all contact with his parents.
I don't think it's helpful for you to focus on their treatment of you & wonder if it's gaslighting etc. All that does is keep you enmeshed in their dynamic.
Instead - look at their behaviours.
They are backing their son in underpaying for their GC.
They are threatening not to see their GC unless you back down.
THAT's how upset they are about seeing them. THAT's how much they love them.
They are emotionally blackmailing you with your own children.
They are prepared to not see their DC - in effect punish them for their son's negligence.
These are NOT people you want to be around, & you doubly do not want them around your children.
They are arseholes who are manipulating you & DS in order to save their son a bit of cash. Cash meant FOR YOUR CHILD.
You should take them at their word, & let them cut themselves off from you.
But make sure you block them - they are bound to change their minds when they next want to play mindgames or to see the GC that they think nothing of using as a pawn.
Don't engage with them again, don't engage with your ex & let the CMS do its job.
They’ve said it that it’s my fault they my DS will now not have a relationship with them.
It's not your fault. It's fully theirs.
THEY made the no contact threat - you have done nothing wrong.
Maybe if DS's useless dad had bothered to ever have a relationship with his child, they could have seen DS via him. But he didn't, so tough nuts. The fact that they choose not to see his wrongdoing, but scapegoat you for wanting him to at least pay for the child he walked away from shows you exactly what they are. These are the people who brought up your ex, & enabled him to be a selfish twat while they blame everybody but him for his actions.
STAY AWAY FROM THEM. Please block them on all comms today.