My daughter broke her collar bone 4 days before we were to travel to DLP, as you will probably be aware there is no cast for this break so a blue sling it was. The doctor fastened it with a safety strap & said enjoy. She wasn't allowed on 'bone shakers' as he put it so rc racer etc but she went on everything else & CM didn't sat a thing. Just followed your doctors advice, take a letter if you want as a back up but I think it will depend on cm on the day & how they view the break. If he wears a coat & it doesn't show he will probably get away with it.