Are you concerned about the new FOP ride? From what I've read he should be fine fitting on the ride with the AFO, unless he already has particularly overly-large calves. There is a thread pinned near the top of this forum about accessibility for the new Pandora rides.
But to answer your question...no, I would not leave an AFO in an
ECV basket. People do dumb things and I wouldn't trust that someone thought it had value to them. Bring a string bag or something so you can carry it with you if he needs to remove the AFO.
Enjoy your vacation!