The first step is to get him checked out by the vet, both tacked and untacked. When there is a problem, always assume it is either because the horse cannot perform the way you're asking him to, or else he doesn't understand.

if everything checks out with the vet (I am thinking that it won't...this sounds like a physical problem if it's progessing like that) then an experienced trainer is in order. No sense in you getting hurt while trying to learn how to work with a bucking horse for the first time.

I agree. Check him physically.

After that, I would work him in the round pen BEFORE I got on him.

Also, what is he on for grain? He could be getting excessize engery from that.

sounds like he has a sore back. either from poor saddle fit, or muscle soreness, or spinal misalignment.

have your vet check him, then have a chiropractor and massage therapist out.

I agree in that the first thing to check is saddle fit, back pain or generalized muscle pain. This will take you on a trip or two to a good equine vet. You may need to try chiropractic work as well, have a good saddle fitter evaluate your horse.
Next, have his mouth looked at as well, perhaps he has pain there that intensifies with the bit.
But you will also have to consider if your horse has learned a method that has been successful for him to rid himself of discomfort or work. Perhaps he did have a pain issue, but even if it is cleared he may have learned a very successful bad habit. How easy will it be to stop him from bucking if he doesn't have pain? Not very.
You might want to try roundpenning him before riding. But I wouldn't just round pen to exhaust him, although some fatigue won't hurt. More than that, I would round pen him to teach him respect and control. Round penning, down incorrectly, does nothing but make a horse frustrated and angry. Consider how you'd feel if someone put you in a pen and then ran you around and around and around until you were beyond exhausted. You'd be angry at the situation, and you sure wouldn't want to go in the pen again. But if you round pen him by directing his feet, having him move up and down in speeds, come into you in the pen when asked, and then also use the pen to sack him out and accustom him to a variety of strange and interesting objects, the pen can be a place to play, get out your extra energy, to learn respect for the person, and even have fun!
I really recommend buying Clinton Anderson's DVDs on round penning. He also has a "Problem Solving" series that might be helpful, I know he discusses both bucking and rearing on that DVD set.
Also, you did not tell us how much turn out time this horse gets, how rich of feed he gets, how much time he spends with other horses, and how often and long he is rode.
I would try to keep this horse in turnout 24/7. He would be better with other horses, then he can use his play drive with them instead of with you. Unless you are struggling to keep weight on him, you probably could totally eliminate all grains. Some horses dramatically improve their behavior when off all grain. And this sort of horse probably would benefit from being riden 5-6 days a week. We think we've worked our horses hard if they are rode for 1 hour a day, but consider a wild horse travels 15 miles on average each day, but not at a run. Most of their activitiy is at a steady walk. Since we cannot ride our horses this much, turnout in a large pasture could help.
Lastly, consider using a different bit to help keep his head up perhaps, but not one too harsh. Perhaps a martingale would help you keep his head up so he can't buck so easily, but as you see, that could easily turn to rearing. You could try sending him to a good trainer. But it is possible that this horse will never totally quit bucking. It has worked for him, he has a long term habit of this, and he may never be reliable. Perhaps he isn't the right horse for you.
I hope you are ALWAYS wearing a helmet when you ride. Landing on your head sets you up for a head injury and possible long term problems. Riding isn't worth risking your life. So good luck and keep safe.

Will James, the author of "Smokey the Cow Horse" said he wouldn't give anything for a horse that didn't buck. He would probably have said that you have a good horse.That may have been the way back then but now horse ways are different. As many of the others said: Have your vet check him out. I could be an underlying medical problem. Check the fleece side of your saddle and check your pad tosee if something could be sticking him. If he checks out medically it more than likely is the saddle fit, especially if he just started doing this.

If you haven't had a problem with him before then get him checked out by your vet and if your vet says he is fine then check to make sure the saddle and bridle fit okay and make sure that there is no stickers in the saddle pad and if that does not help then you finnaly think it is to much energy whenever you feel him tense up getting ready buck then pull him so he walks or whatever in a tight circle to get him focused back on you and keep him doing that until he loosens up though if that doesn't work either you should get a trainer to help you