For starters, he may be bopping his head because he's dropping his inside shoulder from too much contact on the inside rein. In english riding, your outside rein is always on the horse(contact, not lying there), used like a sponge. Your inside rein, when teaching the horse to come round is a give and take movement in time with the inside hind leg. When it comes under, use the inside rein then release. When its released, use your outside rein, squeeze the water out of the sponge, then when you take your inside rein, let the water back in but keep the contact always. Is he quite a cheeky boy? How long has he worked in an outline like this for? May be a lack of muscles, if its cheeky, every time he raises his head reprimand him with a pony club kick, don't pull the reins. Dressage riders goals are to have contact with the reins, but using legs and seat to control the horse and to move away from the hands. Try and soften him up, what bit do you have in? If he's being stubborn, try a narrower bit. And legs, legs and more legs! You'll have legs of steel in no time, hope this helped!