If your horse has had his teeth floated recently it could be how you hold the reins and use them... When i was having a private lesson the instructor kept telling me to half halt (pull one rein) to have him in a dressage frame. Slip kept throwing his head up not because he is disrespectful but because he was reacting from pain now i don't have halt at all to get a frame i just have contact. Try and be more aware with what you do with your reins, and don't let him pull the reins out of your hand or he will develop a habit of leaning against you and throwing his head up. Thats just my experience with head tossing it was my fault all along, so just keep aware of what you are doing with your reins being to harsh is awful, and letting him lean and pull the reins out of your hand can develop bad habits.
The foolish reject what they see, not what they think,
The wise reject what they think, not what they see.