My very first rifle, other than the traditional .22lr, was a .243 but I was significantly younger than your grandson. My twins are proud carriers of 7mm-08's that the old man built for them. They are also a good bit younger than your grandson but are both built like small tanks. Recoil of the 7mm-08 is just a touch more than .243 but it's a caliber/cartridge that they can hunt anything up to and including elk. Whereas the .243 would be on the lite side for elk. Not saying you couldn't use one for them as one can kill an elk with a .22 but one shouldn't for ethical reasons. But if he is going to be limited to whitetail for a few years unless grandpa takes him on his elk hunt then a .243 would be a fine choice. But if grandpa is a super nice grandpa and might take him on one of his elk hunts, I'd have a 7mm-08 in his hands loaded with some 139 grain hornady interlocks with a range out to 250 yards for elk. Guilt trip delivered, my job is done.