As he maintains his lumbar spine in an extension position and then trying to rotate in this extreme position he is stressing a certain motion segment. Teach him to play golf with a slightly flexed lumbar spine. As he develops a more neutral spine his ability to rotate painfree will probably improve. Mobs/manips will only have a short term effect (as in most cases of chronic problems from my clinical experience) if you don't address his habitual movement pattern first.
Hallamshire Physiotherapy