In my own experience - if he doesnt want to pay - he wont. My ex walked away from a long marriage, we had four children, he went self-employed and never paid a penny. It was a struggle - but it was his loss.
At the end of the day, they have left the most important thing in their life - their own child. My advice to you would be - wash your hands of him - walk away - let him live with his own conscience. Look after your little boy, and life will get better for you both, and easier as time goes by. Good luck