My older kid (DS nearly6) is extremely inflexible. Any surprises, smallest changes to routine will lead to tantrums. We have recently started occupational therapy to overcome it. The OT also includes a listening program called ILS (integrated listening systems) and I feel even after a few times that it has made a difference already.
I like the sabotage program, we have used that a few times as well with the favorite cereal having run out :*) or mistakenly putting the wrong ingredient on the sandwich (when he was hungry and we had to run out of the door).
I am so glad that this forum exists and I am not feeling that I am the only one that has a kid with this extreme stubborn/ inflexible behavior. :*)