possibly have a massage therapist look at him... they work wonders.
But other wise, in the walk start flexing his neck to the inside. Do it two or three times in a row and then see if he'll soften on his own. If he gets stiff, do it again. Do it in the walk until he relaxes, then up to trot and do the same.
I have a TB that gets nervous about the hammock outside the ring... and the rolltop... and the barrels at the other end... and the fact that he got in trouble for spooking at everything else... so this exercise works well for him :)
"The white horse moved like a dancer, which is not surprising: a horse is a beautiful animal, but it is perhaps most remarkable because it moves as if it always hears music."