Wiffleballs. He can play all day long with his friends, it's fun, and even travel teams in h.s. use them.
I fondly remember when 3 friends and I were coaching 6--7yo LL teams. We would compete amongst ourselves coming up with new ways to make ground balls, fly balls, etc fun for the kids by using competitive games to keep repetition from being boring.
I think the best I came up with was a whiffle ball game I continued to play for years with my two sons in the front yard of our house.
The name of the game is One Strike. You pitch a ball to the batter at the maximum appropriate speed for his age and ability. If he hits it, you give him a single, double, etc based your judgement of the quality of the hit, (there are no outs for a ball hit fair; in our yard, a HR needed to go over the house) and you keep a running tally of men on base and runs scored as you continue to pitch. If the kid misses or hits a ball foul, that's a strike, he's out, and it's the next kid's turn.
You can play with one kid, or a bunch, but 2 or 3 is best for a fast-moving competitive game that the kids will keep playing until your pitching arm falls off.