I'd go with a nightcrawler under a slipfloat, for simplicity & ease of use. Might not be a bad idea to use a circle hook, too, since all he'd have to do is just start reeling in (no long distance hooksets necessary).
If he's a fairly accomplished 8yo angler ... you might consider using a slip sinker/hook setup (like for Texas rigged worms). Then it's your choice of whether to lay the pole down, sideways to the bait, and let him watch for the rod tip to start bouncing/bending ... or cut a Y fork off a tree limb & use it to prop the pole up, and let him watch for the slack line to start straightening out.
... luck2ya lil man !!