I am certainly not an IEP expert, but I absolutely would be raising this concern to the school. At the very least, he should be having make up classes for the ones he missed. They can't just say "oh well, teacher was busy. So sad too bad." I can't speak to the math class, as my son's IEP isn't for classroom stuff. He gets ST, though. And his IEP says x sessions per month. And he gets those number of sessions regardless of what else is going on at school. When there are field trips, school activities, scheduled closings, etc., his schedule is altered that month to make sure he gets his sessions. He didn't get all his sessions in February, because of all the snow days. I know a friend who still insisted that the school giver her kid all his sessions despite all the crazy weather, but I wasn't going to go to the mat for that.
Your school is very intentionally and be design failing to meet his IEP. I would sure as heck be asking questions as to when he will have makeup sessions to ensure he receives all of his services this month (or week, depending on how the IEP is worded)