Honestly, if this is how manipulative you are over the relationship ending, how were you during the relationship? You are being incredibly selfish, entitled and straight up greedy. You're capable of working and you work, if you're struggling on what you earn - begin looking at ways to increase your income. You are receiving far more than what the law would state you are entitled to yet you are still complaining and are ungrateful, it's pathetic.
He is entitled to leave you, he is entitled to start a new relationship - just like you are. You cannot keep someone married to you that no longer wishes to be with you - why on earth do you think he isn't allowed to leave you?
Considering your own words, your ex is being more than fair to not just his son but you too. The ONLY person making things difficult is YOU and you are refusing to see what implications this will have on your son which shows that his interest is not your priority. All you care about is how much financial gain you can get from your ex.
You have not paid a penny towards the mortgage despite earning, why do you think you're entitled to so much? Goodness, I almost wish I knew your ex so that I could advise him to get a solicitor, stop paying and go through the courts and arrange a plan legally as right now you are taking advantage and it is wrong. Imagine if the positions were swapped and your ex was doing what you are doing to him to you!
In regards to the new girlfriend turning him against you, taking into account how manipulative and greedy you are, I don't think she has to say much, you make yourself very unlikable with your behaviour.
The priority is your son, and he belongs to you both. His best interest is what matters and surprisingly I think only your ex right now has his best interest which is why he's doing everything he possibly can for you both, whilst you just sit bitter and trying to make things more difficult.
Pathetic, just pathetic. So many broken families and so many women do so much just to give their child the best, who receive so little but still encourage relationships with the father of their child, and then there's people like you.