I wouldn't even bother trying to cover it with sand, he'll probably just dig it out again. A much easier and less time consuming way to keep the light from bleeding through is to just get some of the cheap walmart aquarium background and tape the black side to the bottom of your tank. That should block out the light coming through. If thats to thin and still shows thru, you could always tape some cardboard behind that to block it out better.
But if you really want to cover it back up with sand, just throw your LR
in a Tupperware tub and fill it with some tank water to keep your rock submerged. It will be fine for a while as you deploy your new sand and get it smoothed out. Then just place the LR
back in the tank and refill with either fresh SW
or the water you pumped into the Tupperware box.