Hi jools987,

Interesting - how did your rider go?

If Wikipedia reference-linkMRI was negative, then to me it is probably a movement dysfunction / motor control issue.

He may be overactive causing excessive compression of his spine and thus pain and tenderness like sore bones in a non-specific way

He may be overactive in the hips causing increased flexion to occur in the back and so he might have a "hinging" vertebral joint which is excessively loaded or moving - PPIVMS and PAIVMS should be able to let you know about this

He might have other problems in his thorax such as thoracic rings laterally shifting/rotating causing failed load transfers. This could cause his long thoracic extensors or obliques to become overactive can so cause low back pain...the low back is the source of the pain but the cause could be failed transfer elsewhere.

Any, just some thoughts

Cheers!