When I read your first post I was going to say....get one of those runner ones... but I can see you have already done that!
I would have a quiet word with the teacher, class teacher or head teacher, and point out your concerns. Schools have the rules for good reason, but in this instance, I would try and get them to make an exception. In my opinion, it is not the danger he faces during PE, if thr teacher is aware etc, but the fact that he is human, and will, at some point, forget to put it back afterwards...and this cpuld be a problem.