I'm all for teaching kids to shoot and exposing them to the great outdoors but certain things don't go together. There's a little bit of "nature's input" involved with this. By worrying about him being able to handle a .243 you're brain is telling you it's too big for him yet you heart keeps trying to have you back door the whole natural progress of things.
Kids, or any shooter really, that can't handle the said recoil of a certain gun and/or cartridge shouldn't shoot/use that gun/cartridge. Quit trying to gadget things up and turn an 8yo into something he's not or a .243 into something it's not.