Yes, probably most of the rides except for perhaps water rides and the very extreme, high action rides. Ask a Disney cast member when in doubt.
Sure
Yes only a few rides have water at magic kingdom.
For the most part he will be fine. If it is a full cast above the elbow, some of the more adventurous rides with the pull down restraints might be tough, but I think you can still ride them.
Depending on what ride I am sure they can help you out they are more than accommodating
For some rides it will be fine. There are rules posted for every ride. I would call guest services for more clarification.
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