Do you remember the kits where you make your own bouncy ball? I had one as a kid, and I found a kit for Levi, and gave it to him for his birthday.
Its a quick project that he can do almost entirely on his own, and it gives a fun result! I got this kit from Fat Brain Toys.
Here is what you get:
Put the mold together, and pour in layers of powder (like sand art). Make sure you tap it gently after each layer, this ensures that you won't have a gap at the top when you set the mold.
Then put the whole thing in water for 1min, let it sit for a little bit, and un-mold. The kit I had as a kid required hot water, but this one is nice because you can use cool water, so the kid can do it himself.
He loved the bright colors and the rainbow/tye dye effect.