have you tested the endurance of his deep neck stabilisers (e.g with a biofeedback cuff)? I've got a young patient at the moment who gets very bad headaches- absolutley no physical signs other than weakness of her deep neck stabilisers and tension in her occipital muscles (she's had all the usual medical investigations), she's had accupuncture, mobs etc previously with no benefit. I've just started her on a neck stabilisation programme this week for her deep neck musculature...hoping to see an improvement over the coming weeks but it's early days at the moment so I can't comment on whether it will be successful for her....but you could check it out on your chap.