My son is 9 years old. He has been swinging the Easton S3 29/19 for the last 2 years. I think he needs a heavier bat. Looking at getting him at least a 21 oz. What Easton bat do you recommend?

At that age, I recommend you don't go any heavier than a drop -10. Have him put one are out to his side with fingers stretched out. Measure from the center of his chest to the tip of his longest finger. That is the length of the bat he should be swinging, +/- 1/2 inch. If you want a heavier swinging bat, go with a -10 that swings a little heavier. I recommend these three:

https://www.99bats.com/collections/barrel/products/rawlings-quatro-pro-10-2-3-4-usssa-baseball-bat-ut9q10

https://www.99bats.com/collections/barrel/products/easton-2018-ghost-x-10-2-3-4-baseball-bat-sl18gx10

https://www.99bats.com/collections/barrel/products/easton-2019-ghost-x-evolution-10-2-3-4-usssa-baseball-bat-sl19gxe10