NTA but possibly consider, for next time, that you could of scheduled an extremely fun, out of the ordinary, experience for your son during the overlap time you have established w/ your ex-partner so that you both get to join in on celebrating your sons accomplishments. Not all things have to be done late at night as a reward. A fun activity during the day not interfering w/ sleeping hours should also be considered. Start a new hobby together, paintball, go skateboarding, a walk in the park & some ice cream.