WHAT TO DO: My horse BUCKS madly

First of all you must understand why this happens. It might derivate by back pain or illnesses (for example: Cold Backed Syndrome). 
So, the first step is certainly a vet visit. If everything's ok with his health and nothing in its eating habits has changed, you have to concentrate on yourself: is your position appropriate? Do you use correctly reins and stirrups? Do you have a good balance and not lean ahead or back? If also these are not the causes, the horse may be scared by whip or spurs. In order to understand if this is actually the real problem, try to eliminate these advices for some days and see if the behavior changes. If this is the problem, you may have to integrate the presence of these objects slowly (read the article: Overcome the fear of WHIPS). 
if this is not the cause and even without whip and spurs, he continues bucking madly, you have to search for something else. 
There are two other huge and frequent matters: 1) the horse doesn't tolerate the saddle and being ridden; 2) the horse doesn't let it out enough and when out of the box, he needs to vent. 
The second one is probably one of the most frequent causes: the horse lives in a box and stays in there for too much time and when taken out, he gets mad. Of course the solution is simple: he needs to avoid getting bored, so take him out more frequently, not necessarily riding him, but lunging instead. 
If the horse doesn't tolerate the saddle, you need to re-start the saddle introductory process from the beginning.
-You might have already done this, but if not (HUGE MISTAKE!) gain its trust.
-Then lunge him, without harness.
-Start slowly to enter the saddle pad while lunging him.
-Enter the saddle only when you are sure that he is ready for this.
-Remember to reward each step he does correctly and also remember to give him its time. Each horse has personal needs and forcing him will make him develop a bad behavior and not overcome the problem. 
-At first DO NOT RIDE him. Let him get used to the saddle first (remember to take away the stirrups or they will hurt him).
-When he gets used to the saddle's weight, make sure that it is ready for yours too: let someone hold him, position on his left side, put you forearm on the saddle and lift by putting your weight on the saddle. If he seems calm and doesn't move, give him a treat and while someone keeps holding him, get on the saddle. This has to take a couple of minutes and not more.
Get off the saddle and repeat this step for almost 5 times.
-The day after you can try to walk him. Before riding, lunge him for almost 10/15 minutes and the ride walking for 5 minutes. Increase the time and also trot and gallop day by day (if he doesn't seem hurt or nervous). 

Remember that there are lots of other problems that may be the cause of his bucking and these are just a few. If your horse has this problem, talk to your vet and trainer. 
This is a simple, natural approach and it takes time and patience, but is not invasive.

Saba

Credits for the image: https://it.pinterest.com/pin/161425967871820694/