If it's an isolated incident, and he's genuinely trying to see you soon and not just continually putting you off, let it go. People really do make mistakes, and when you're juggling a lot of stuff you can't be thinking about even the things and people that are important to you all the time and so you can slip up.
However, if it becomes a pattern, that's a big problem...
If you feel you need to talk to him about it, just tell him that you know mistakes happen but that for you it can be really saddening to be left hanging because you feel disregarded, and ask if there's something you could do to help with scheduling (follow up text to confirm?) in future.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.