has he got any wolf teeth coming through if the bits catching those i wouldnt be surprised if he kept going up, also does your saddle fit him? does your bridle fit him? does your bit fit him (not just in length but check if you have a loose ring that its not catching his lips, or the mouth piece is sitting incorrectly or that youve got a thick mouth piece in a fleshy mouth that would create a very sore pressure point) or does he have any sores or girth galls that could explain the sudden explosion if he was just wearing a saddle. id have a good look at him make sure thers nothing obvious thats been hidden if not then id get a back man and a dentist to give him a realy good look, it could be somthing simple but you wouldnt want to make it worse.

if hes been given the all clear by everyone, then he probably wants rebraking slowly. Although you havent said if youve bought him broken in or you did the breaking in or if hes been good as gold but then hes acted up recently. Last thing did you get him vetted when you bought him?

sorry for the questions but thers wall too many reasons that horses kick off with tack you just need to narrow it down. hope this helps :)

Perhaps the bit is hurting him. :)

he may be uncomfortable with the bit or the way it is in his mouth. make sure the bit isn't too tight in his mouth and that he is comfortable with it in his mouth.