one of the byproducts of a deep hook, like Don said, is a twisting tension on the string, if your nock is a snap on, then try this...nock an arrow then show him that by rolling the string the arrow will go that way that you twist. when he starts to draw have him place the string on the second joint instead of the first, then as he starts to pull let the string roll down his fingers and settle into the first joint. this has the effect of tensioning the arrow into the bow. someone may want to argue this, but it has never affected my accuracy, and i have never had a problem with arrows coming off the rest. it has helped my kids, too. i can even cant the bow in the open direction and not worry, usually.
also, everyone above is right its not the radius, but you might try inclining the shelf slighty from outside edge up to lower towards center. (keep the radius) this will help keep you on the shelf too, as the arrow will have to move up hill to come off the shelf.