Thanks everyone. Good points.
Yes, he was once told he was using just his arm too much. Always a big kid, he tends to cheat, but that's never mentioned anymore.
yes, some are just very gifted, no stride seems to be needed.
Yes, he needs core exercise. What got me was how both coaches, the most knowledgeable that have seen him, simply say "It'll come" He's also been told often that he needs to grow into his legs.