FJ is fine as long as he can find a way to hold his arm. It's Dragon Challenge (and Hulk) that he absolutely can't ride.
We hit both USH and UO this summer after my kid broke radius and ulna and was in a 90 degree at the elbow cast up to his armpit, and those two were the only ones he was forbidden to ride.
He did take full advantage of the tester cars on RRR and FJ to see how he would position himself! And we brought our waterproof cast protector for the water rides.
Here is his cast to show you what he was dealing with. (I still cry when I see him in the cast, but even when he had it on he dealt with it soooo well, even though the actual timing of breaking his arm was horrible, since it kept him from performing in a closing show of a week-long tap dance convention) He was dealing with it so well he figured out how to play Transformers videogame at RPR's arcade.
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Realized that one of the signatures was easily visible...in case you already noticed it...yes it says "this makes me happy". It was sort of a joke. The woman that wrote it is highly sarcastic; like she needs a warning label LOL. But at the same time, she broke BOTH of her arms at the *same time* when she was about 10 and living with her dad and stepmom. After the first bathroom visit with her stepmom awkwardly helping her, she decided she would do it all on her own. She kinda sorta feels that all kids should deal with adversity like that, to learn how to deal with life. So she kinda meant it, that it made her happy, because now he had to deal with adversity. But she's also massively sarcastic.
I was not unhappy when they changed that cast out a few weeks later...got old explaining that one!