Mystery illness — pain in abdomen, back, chest, and head; no rash

Hi all, I've been hunting through these incredibly helpful discussion groups for the past couple of days, looking for something that might match the symptoms my father (73 years old) has been experiencing for the past 1-2 months. Hoping this rings a bell for someone, as I haven't been able to find anything like it so far, and the doctors are baffled:

• pain that started in the abdomen but now also incorporates the head, chest, and back
• NOT a burning, stinging, or itching pain
• rather, the pain feels like an inflammation
• spans all areas of head/stomach/chest/back, i.e., not just on one side or in one area
• the pain roams around; sometimes it might be in the abdomen and head, sometimes more in the head, sometimes just in the back, etc...
• the areas hurt even when they're not being touched, but they hurt even more when touched
• no visible rash
• lying down and not moving eases the pain slightly, but not completely
• no fever
• no other gastric symptoms (e.g., nausea, vomiting)
• the pain increases or decreases in waves, but persists at a certain baseline. That is, my father hasn't had a pain-free day since this begun
• a similar "episode" of this pain occurred some months ago, but faded away on its own after a couple of weeks
• tests conducted so far include MRI, CT, X-ray, ultrasound, bloodwork, stool and urine samples; all have revealed nothing
• my father is currently taking Tramadol daily to manage the pain

Would welcome everyone's thoughts on the matter! We're off to see the gastroenterologist for a follow-up today, but so far everyone is completely baffled. Thanks in advance, and sending courage to everyone struggling with their health.