I agree with alophysio. Note mobilising a "stiff" upper Tx segment is likely to be moving cervical segments at the same time (into extension). Also the dural connection up from around T4-T8 will be affected during your mobilisation.

One cuold also condier the symptoms of thoracic outlet syndrome and the vascular symptoms from this type of presentation. This includes paresthesia in an ulnar distriction and even weakness in the 4th and 5th digits.

Are there any other signs or symptoms, e.g. systemic considerations. Is he a smoker. Have you got a plain chest xray? A complete physical exam is exremely impt at this stage