So, OP... WHO is the one with the problem here? Is DAD the one wondering if he gets make-up time for the fictitious time loss, or is it you who has a bug up it? (I can guess.)
I commend Dad for agreeing to take his kids to this concert so that they can perform pieces that they have likely been practicing for a period of time. That's what a GOOD parent does. Even if the concert had NOT been on his time, as a good parent - he WOULD make a point of being there to support his kids.
What a lousy parent does is use such activities/events to stick it to the other parent of their children. This is not an activity that occupies regular amounts of his time - which is what I expect the order was intended to prevent.
What a truly lousy STEPparent/SO does is use such activities/events to drive a wedge between a parent and his/her children - which is what you are trying to do by trying to create a "situation" between your husband/bf and his ex. That's something you should be deeply ashamed of.
Your husband/bf's kids are lucky that they have a Mom who (apparently - although I am sure you will now dig up a ton of examples to the contrary) goes out of her way not to interfere with Dad's time with their children. They are also lucky to have a Dad who supports their activities.
Being a parent includes sometimes giving up the time you'd like to spend with your kids allowing them to do stuff with friends, with a group, on their own, and often being little more than a chauffeur. I cannot tell you how many hundreds (if not thousands) of hours I have spent driving one kid or another or both to rehearsals, practices, games, concerts, performances, competitions, auditions, interviews, tours, tests, classes... you name it. We make the most of the time we have together - it would never occur to me to limit their interests or abilities by refusing to do so or demanding some sort of make-up time. (Though I'm not sure who I'd demand it from!)
What will Dad (aka you) do when the kids are older and they have to take the PSATs (for example, and only given on one date, with very limited exceptions) on HIS time? Will he demand make-up time from Mom? Or will he be pleased that he gets to be the one to make sure they have a good dinner the night before, a good night's sleep, a good breakfast... have all their pencils, registration info, etc.?
Being a parent requires much more than just "fun" time for Mom/Dad. It tends to require a lot more work than that. And if you're not willing to allow Dad to do that work - and reap the pleasure from it? You should walk away now.