This horse isn't afraid of anything, except maybe work. The horse knows you all do not have a clue as to what you are doing and has decided to run the show.
The problem isn't that your kid used a whip, the problem is that he doesn't know how to ride. Or how to handle a horse.
And I don't know that a refresher course is needed, as if this horse was used as therapy only? Horse might not know much at all.
But a week at trainer's is not going to do anything, except maybe convince the horse to behave at trainers. Does not mean it will do a thing you want it to.
I'd send horse off to trainer for a couple of months, a REAL trainer, that is, someone who really knows what they are doing, not just someone who "rides a lot, and has horses" as so many people think that is a "real trainer."
You need someone that also will give all of you lessons in horse handling both from ground, and in the saddle.
There is not a horse alive that won't do this, but there are horses out there that won't take as much of an advantage as this one seems ready to, IF this horse is even trained well to begin with.