How do I handle parent not following a court ordered time share plan?

Changes to the order are not permitted unless both parents agree. That means if he wants less time your options are to a) agree, b) not agree and advise that he either takes whole weekend or no part of the weekend. But he can only return them early if you agree. Since this may create issues with your kids, it is possible that you may start denying him the shorter weekends in order not to be stuck having him return them early.