I think your simplest way through this is to renege on the offer to pay CS through college and pay support for the 16 y.o. for the next two years.
If you agree to support through college, you set a precedent and end up paying for both. Also support amount and duration will be more difficult to negotiate because support during college is outside the realm of child support in most states.
You are right, only the parents' income should be taken into consideration when figuring child support amount.
I'm advocating he work to minimize his child support obligation. I'm not advocating that he not support his children through college. He always has the option to contribute directly to their education and he should if at all possible.