To me it looks like a hoof issue by the way he's short striding on it and the head pop. That isn't the same as a head nod on a gaited horse or it would be rhythmic. It could be an abcess coming on or something else. If you really want him do get the vet check done. We've heard too many broken heart stories on here from those who didn't. If you don't want to do a vet check, wait about 6 weeks and if he's till for sale, try him out again. If it's an abcess it's usually run it's course by then and he'll be fine. Since he's white, I wonder about his age as most greys are white in their late teens or later.