It sounds like the doctor has assessed correctly in that it is musculoskeletal.
Not much is in that area but bone, muscle.
Below that area is the stomach--but the rib would not be tender itself if you had problems with the stomach. Ulcers and pancreatitis would present with abdominal pain, nausea, vomiting, weight loss. Pain would likely be worse after eating, not related to positioning/movement.
The chest xray is pretty good for looking at ribs, you may want to have your doctor do specific rib xray to assess the actual rib area that you are having pain. If it doesn't get better and the pain is specifically on the rib itself, you could have a bone scan done. This will tell if there are any abnormal spots on the rib (small hairline fracture, tumors, etc) causing your pain.
Reproducible lower chest wall pain is likely muscular or bony. Nothing else is in that area.
Chest xray ruled out lung
Only other thing to do would be blood work to look at pancreas enzymes but again, nothing in your symptoms suggest abdominal problems.
If you have more questions or more to add, let me know, I would be happy to discuss.