Abortion is an option, not a requirement in a surprise pregnancy! A woman should not have to shoulder the full financial responsibility of a child just because the man who helped make it decides he didnt want it, especially when that man was that bothered about pregnancy when he was getting his rocks off.
Every adult knows that sex can make babies unless you do something to prevent it, especially an adult who has fathered other children.
The OP didnt use contraception because she didnt need it as far as she knew, he didnt because he didnt give a shit. Frankly I have no sympathy for him, and yes he should pay.
Just because she decided to continue the pregnancy that he didnt want, which after all may be the OPs only chance to be a mother, does not make her unreasonable to expect him to contribute.
I dont know what they teach the boys when the girls are getting "The Period Chat" at school, but I hope that theses days its "either accept that when you have sex you may become a parent with all the responsibilities that entails, or dont have sex".
However, as an aside OP, you really need to insist on condom use in future. Fertility issues notwithstanding, you are risking your sexual health every time you dont and I hope you have been tested since your son was born.