Would this be worthwhile? Would he find it interesting or would he be bored out of his gourd?
Probably a little of both.
It's interesting to sit in an actual courtroom, and to be able to observe how it looks first hand, and how people are required to behave, like rising when the judge enters the room, and the formalities of the situation, and how all the various participants and court personnel perform their jobs, for instance. Watching these things on movies or TV is never the same as actually being there.
When I was 11, I would have loved to go sit in a courtroom, particularly if my dad was involved in the proceedings. I loved to go visit different kinds of public buildings, even different kinds of movie theaters, and I liked to observe first hand things I had read about. Of course, I didn't get to see places like courtrooms as often on TV or movies as Mo probably does, but still, it's always different actually being there.
And I would have enjoyed watching my dad in that kind of situation, because it wouldn't be the way I usually saw him behaving or interacting with others, so I would have found it interesting to observe just because it was my dad on the witness stand.
And, it's good to learn that things aren't always as they seem in movies. Real life trials move slowly, with interruptions, and lots of pauses, and attorneys can't stand right next to witnesses just so they'll fit in the camera shot. So Mo can learn that too. He might even be interested in how the court stenographer can record all the testimony so quickly, or what the court officers are required to do, and maybe he'll get the chance to speak to them.
I'm sure Mo will find some of it boring and some of it interesting. But he'll also be getting to experience something that most of his classmates can't see first hand and that's important and worth skipping a day of school.