Just because support was not ordered at the time does not mean you cannot modify the order to include it, particularly once you become the parent with primary custody.
He's going to keep yanking you around as long as you let him. What he doesn't realize is that if you obtain primary custody over your younger son then you can pursue child support, probably support over the the guidelines due to child care needs and specific medical attention for his developmental disability, lose the right to claim your son on his taxes while you would likely be able to obtain access to a number of assistance programs and tax credits. It's almost assuredly financially disadvantageous for him to pitch this fight with you. The stupidity in his position is that if you go pursue a modification to include this support it's too late for him to argue that you shouldn't have custody (and therefore not need child support) because he's already made the argument that you should have custody. You would want to modify the custody language to specify that you are now the custodial parent and pursue all the support for which you are entitled.

