In May. His ex is wanting him to either continue paying child support if their son decides not to go to college. Or continue paying the same amount to the mother if he does decide to attend college (either way the amount will go unchanged) Does my husband have to contnue paying child support (back child support) if his son decides not to go to college. The court agreement was to pay child support until their son turned 18 and graduated. Does this change because there was a period when the support was not being paid?

Back child support has to be paid till it is all paid, the "child" can turn 30 and have kids does not matter, it is a debt that is owed. So any and all support has to be paid.

Current or new payments may not have to be made if the child does not go to college, but all payments for any back support has to be made.

... The court agreement was to pay child support until their son turned 18 and graduated. Does this change because there was a period of time when the support was not being paid?

In all likelihood, no.

It is posslble, I suppose, that the court would require post-18 CS payments because he is behind, but I have never seen it done and would be surpised if it were.

The answer is yes. The child support payments continue until the child support is paid off. So when the child turns 18 and graduates then the support ends as far as reguler child support. That part will be over. But the payments continue and are applied to the arrears. Depending on the state you live in those arrears even small ones if they were long ago may have interest tacked onto them. There may be a huge balloon amount. It is in his best interest to pay off all arrears as quickly as possible.