OP,
It is regrettable that your husband lost out on a significant part of his son's childhood. Alas, that can often be the results of random sexual encounters.
However, none of that excuses his financial obligation to his son ,nor does it justify his choosing to not work. Working fathers, as is the case of most nuclear families - and many if these nuclear families include working mothers and fathers, do not lose out on their children's childhood because they work.
His unfortunate earlier experiences just provides a handy excuse for not doing the hard work of finding a way to provide adequately for both of his obligations. What if rather than the 250 to the mom that you were having trouble paying it was 250 towards your mortgage, would he call the mortgage company and offer to provide childcare services in lieu of payment ?
The ex and the son are simply viewed as the most readily disposable of his financial obligations and that is unfortunate.
Again, like many working dad's with multiple children, he could join the ranks of those who leave their day job and go directly to their evening job. These men love their families enough to actually want to provide them.
Your husband's son represents a life created out of randomness and providing for his future seems no more important and significant than was his conception. It is all about what is most convenient for the adults involved.
OP, I am not criticizing you for wanting to make your life easier. Your husband's son existence and financial security is a cautionary tale filled with examples of adults doing what is most convenient fulfilling their needs and preferences.