Kyle's been doing the bowling league for the last couple of years, on and off. He's 10 now, and started when he was 7.
Most bowling alleys will have 8 pounds and 6 pound balls available to borrow, if they have any kind of kids league anyway. The problem is that the lighter the ball you use, the less chance you have of knocking any pins down. Light balls, when you don't throw them very hard, have a tendency to bounce off pins rather than knocking many down. It gets frustrating for the kids when they throw what looks like a perfect ball and they only knock down a couple.
8 pound balls are fairly common. 6 pound is harder to find. The 6 pounders have more tendency to get stuck in the pin machine, but every alley I've ever been to has 6 pounders. I've never seen anything less than 6 pounds.
For us, the 6 was too light, and the 8 was too heavy. I searched all over the internet and finally found just one place that had a 7 pound. I bought it at www.bowlerstore.com. Unfortunately, they only have it in pink right now, but if you search or call around, you might find one in another color. It came in light blue at one time, that's what we have. Here's a link. The brand is Linds and the model is Laser.
Sometimes I saw these very light balls referred to as "alley balls" - basically the kind of ball that the alley has on hand to borrow. They generally came in 1 or 2 pound increments from 6 to 16 pounds. You need to work with a specialty place to get something like that, either on the internet, or if you have a good sized bowling alley near you with a good pro shop, they might be able to help. You probably won't find a 6 pound ball at a big box sporting goods store, and you definitely won't find a 7 pounder.
I spent a ridiculous amount of time trying to find the right ball for him.
Next step is getting the holes drilled. This is worth going to someone that knows what they're doing. We're crazy people, but we drove an hour and a half to go to someone that was supposed to be really knowledgeable about setting up a ball for kids. He looked at how he threw a ball before drilling, and how he held his hand. Since Kyle's weak and compensates by adjusting himself, I thought having something other than a standard hole pattern would help. The pattern the guy drilled is different than you might imagine - whether it's actually helping or not - I don't know. Just something to think about. I'd ask somebody that bowls in your area, they can point you to the people that know what they're doing. A good ball for an adult can be $200-300, so they don't take something that costly to just anybody.
I did ask Dr. Wong, by the way, about bowling as an activity, since there's some weight bearing involved. She thought it was fine, as long as it wasn't fatiguing, of course.
Wow. That's more than you wanted to know - isn't it?