Drew’s issue with the thumb from the start was gripping so he needs the mobility and strength to bring the thumb toward the palm and fingers. After any surgery you tend to focus on mobility/range of motion and then strengthening. A smaller ball has a smaller circumference meaning it’ll require more mobility to grab onto. I’m sure strengthening is still an issue as it is after any surgery. I see being able to firmly hold a smaller ball as him having the mobility in the thumb already. What’s left is strengthening.