Could it be an allergy to something new in his environment? Have you changed detergents, soaps, or does he have any new clothing?
When I was younger I found a blanket in the back of the closet I wanted for my bed. I washed it in the same detergent I used for my clothes and put it on my bed. Shortly after I broke out in little red bumps all over my legs - they didn't itch, they were just there. It took weeks (and several trips to the doctor) for me to connect the blanket to the bumps. Once I removed the blanket, the bumps went away.
Good luck, and hopefully this next trip to the doctor will clue you in to something!