The pain on rotation is seen very often in rib troubles. Have you tried rotating him from side to side and then force him to inhale maximally? This maneuver can sensitize the test a bit and hopefully point you in the right direction. The costotransveres and costovertebral joints also share innervation with overlying skin (medical/lateral branches of the dorsal ramus), thus sensitivity from a rib subluxation can also produce focal tenderness on the overlying area. As I'm sure you are aware the ribs are often affected by bony pathology a plain films are beneficial in ruling out anything more serious. I very successfully manipulate these types of rib lesions after a full work up has been completed. The procedure is very simple and uses a rolled towel placed over the lesion (bias the side which is problematic) have the patient interlace their hands on the back of their neck. You then with the towel pinned on your belly will reach around taking hold of their wrists. You will have very good control over their body in this position and so even If you aren't game you can still use this set up to mobilize the rib. have them relax into the towel and then deliver a HVLA thrust through your towel to the lesioned site by extending your legs and tractioning upwards. You will likley hear a pop and patients feel almost immediately better if this is the problem and you get it right! I would recommend reading and looking at some pictures in this book before you try or refer them to someone who has manual manipulative training (physio/osteo/chiro):
Spinal Adjusting Technique by Downie, Phillipson and Esposito.