First off: love that you’re doing this!!! Wow, you’re setting your kid up for success.
By 12, I was mowing lawns - tons of them, all over the place. It was awesome. Brutal but lucrative work. And it’s the best work-related learning experience of my life, to this day; I essentially founded and ran a business and it blossomed.
Also: anything. Go around and solicit neighbors, or people he can get to, and people you trust. As someone said, I’d do any odd job that came up. One time, we moved a man’s property from storage to somewhere. Got stung by bees after I put my hand into a nest, but made extra money and didn’t care. (Also learned that pulling your shirt away from your body as they fly at you will save your hide - literally!)
The key is if he can get to customers. If he can ride a bike or walk and it’s safe, then find anything they need. Clean out yards, for instance. Minor car stuff, even. Asking around may well lend itself to opportunities.
FWIW, I carried a mower and tied a weed eater on, then tied a rope to the back of my bike, put on my big straw hat, and pulled that mower all over the area. I’d ride a bike 10 miles each way to my next job back and forth - whatever got me there. (Granted, it was pretty safe…although looking back, maybe not all that safe.)
Another idea: working at places that do more cash business. E.g., skating rinks might hire a kid that age. Or other kid-friendly places that have lots of kids and are mom & pop shops. Just a thought. But he’d have to have transportation - that’ll be your limiting factor. (Unless you want to turn him loose on the internet, and for that, there’s plenty of input elsewhere re: side gigs…)
Best of luck to you and your son!!!