Everyone here has some great points. To expand on those, have you changed your riding habits since you started riding him? Something you are doing could be hurting him physically or could be sending the wrong message to him and causing the attitude problems. I personally am a big proponent of natural horsemanship to resolving problems – if it is the latter, after being checked out by a vet for any physical ailments, of course. There are some great resources online for natural horsemanship, and you may be able to find someone in your area. Stacy Westfall, Clinton Anderson (of Downunder Horsemanship) and Buck Brannaman all have great videos to help you get started, but a clinic or trainer would be best.