I imagine he’s assuming the 11 days holiday is because he wrongly believes that his every other weekend contact should not be disrupted, which is incorrect. As long as you give decent notice you can holiday on his contact time, though would try to avoid excessively disrupting his time.
Moving forward, you make your son available for contact at the time of the collection as per order. Give leeway of 20 minutes, if doesn’t arrive then he loses that contact.
You’re not obligated to now go against the order and be flexible, if it was a continuous situation. The odd time yes.