[Fetowip]

maybe lay wire netting out flat around the slab ...pin it down, or weight it down .netting isn't pleasant to dig thru ,or lie on.
depending on what the legalities are in your state ..maybe consider a PINGG STRING, as a temporary teaching aid ....

get him one of those clamshell thingies they sell for kids.. fill it with the same coarse sand ...place it in the shade ..bury a few favourite toys/treats in the sand to encourage him...