We just saw doctor for older sons knee issues. He noticed my son pronates, although my son's feet are not flat. He told us to get the green Superfeet inserts. Found the best price on Amazon. Still waiting for the order. I forbade flip flips and Converse and the like ages ago, but my son also needed new athletic shoes that fit better, his old ones were too lose.
Doctor also gave us a sheet with exercises for strengthening the leg muscles to help protect the knee. Every exercise emphasized to hold the working leg about 12 inches (or less for a couple) above the ground for about 6 seconds per rep. While helping my son learn them tonight, he kept saying "we do this at the gym but my leg is much higher and I hold it waaay longer." I had to get my out of shape carcass on the floor to show him why if you hold the leg too high you can hold it all day, because it is not working anything! How can a kid do gymnastics for 7 years and not know this?
Tightness makes me wonder if a different type of stretching would help. Would your son consider yoga? (I have been trying to get my boys to do yoga for years to no avail.) Or massage?
I never heard of Xbraces. This?
https://www.thexbrace.com/pages/what-is-the-x-brace Can they be worn in the gym?