I think Elz that your ex probably can't believe his luck. He will know exactly how much you could claim from him and probably spent months sweating buckets that you were going to claim. Of course he doesn't want to hand over that much of his income.
For a man, he found himself in the worst possible situation - affair and pregnant mistress. But he's got away with it. Completely. Has he learned a lesson from this?
Yes, you can behave like a git to everybody in your life and get away with it scott-free.
Why should he? Why on earth should you be alone, with his child with no emotional or financial support? You did nothing wrong, he did. And continues to do. How cruel to not be involved with your son. This is his son fgs!
He's thinking about himself and his wallet, not you or your child together.
I think you should approach him and really ask him how he feels not being involved with his son. How he will feel in 20 years time that he has a child he hasn't see grow up? I think your son deserves some financial support and contact with his dad. How will you feel if he turns round at 18 and tries to find him? How will your ex-partners family feel then with a secret that's gone on for 18 years?
By all means don't go stampeding in like a deranged woman, but I think you do need to change the status quo - because soon your son will be old enough to ask questions about daddy.