College bills come due in the fall (Aug/Sept) and winter (Dec/Jan). If I were in your position, I’d tell my son that I’d be happy to send the college a check for 1/2 the yearly amount agreed upon in the court order (for the fall semester) as soon as the school provided me with a copy of the bill (the financial aid package). And that I’d be happy to continue to send the school 1/2 the yearly amount every semester they sent me a current bill. I would never write a check to anyone without seeing a bill, and I think if you do that it’s a bad example for your son. He may have to sign a paper for the college that allows you access to his billing information, but it’s possible to arrange. If he takes care of it tomorrow, the enrollment fee could be paid by credit card on Tuesday.
If your son thinks he might miss the registration deadline, he can call and ask for an extension. If he and your ex refuse to allow the school to give you a bill, your son can ask the school if he can defer for a year, he can take a gap year and apply to schools that may be more affordable, or start at a cc. But I wouldn’t let him start college by being evasive and failing to communicate the most basic information about his search unless that’s how you want to spend the next 4 years.
I’d also suggest that he ask the school what happens to his financial aid package if there are changes in the number of dependents while he’s in college. Make sure he knows how much you can pay, and if you won’t be able to contribute more if the aid is reduced make sure he knows that too. He needs to understand costs could go up and he should be aware of how much he can expect from you.