^^ as above. Check your saddle, bridle, bit, and get him looked over thoroughly by a physio and/or vet, and have his teeth assessed by a qualified dentist.
If it's not pain/discomfort - he's well and truely got the wool over your eyes.
If he were my horse, I would take out my dressage whip, ride him normally, and as soon as he goes to stop, ask nicely once with my leg, then give him a couple of good cracks over the backside to make him think twice about not taking you seriously.
Yes, sounds 'mean'. But, I would sure as hell rather have a horse that knows that I would take any of it's rubbish, and then happily works for and with me, rather than one who is constantly trying to take the lead because I haven't been a good enough leader.