The military gives you a medical exam upon leaving, mostly to cover their own ass under these circumstances. If you have no issues, the exit exam consists of the base doc asking if there was any problems, taking your blood pressure and sending you on your way.
Most people with even minor issues do use this as the basis for their VA disability claims. Not just like "I'm an amputee", but many lesser disabilities, usually between like 10% and 30%. I'm wondering if failing to mention something as widespread as Fibromyalgia during so would screw him, or if he claimed it and the VA denied it.