The fact that he was prostrated, lucid, but suffered intense pain best sustains the hypothesis of a pelvic fracture. After the initial period, one of the most common causes of death resulting from a pelvic fracture is deep venous thrombosis with secondary pulmonary thrombo-embolism.
Given his condition of being prostrate yet able to think clearly despite severe pain, it strongly suggests a pelvic fracture.  After the initial stage, a major cause of death following a pelvic fracture is deep vein thrombosis followed by pulmonary embolism.