Joint Custody: When a parent doesn't show up for visitation

My four year old son gets so upset when his dad doesn't show up for his scheduled parenting time/visitation. What should I do?This one is pretty straightforward. It's a rare four-year-old who maintains an internal clock and calendar in his mind. It's up to you to remind him what happens when. So simply don't mention that his dad is supposed to come.If he shows up, let it be a lovely surprise.