My Gelding Has Lost His Mind!!

I don't know where you are in experience.  Since you're asking I'm wondering if you could let someone do a tune up on him.  Otherwise, it sounds like MY gelding and apparently yours feels so very good he's forgotten who's boss!  (good job fattening him up, btw) []  It's sort of scary when Zag acts that way but I learned from a stupid teenage boy (mine) that with Zag, I need to push him through it.  He doesn't really direct it at me, so I have to try not to let his misbehavior spook me.  But a trainer recently told me, you don't want SOFT hands, you want SLOW hands.  Don't allow him to pull the bit to get more rein, etc.  I don't know about a tie-down to keep from rearing.  Rearing scares me.  trainer101 will be here directly.  I'll predict she will start with ground work. []

Maybe it's time to back-off on the weight builder or any grains you have him on a little too.  Please be careful, sound like a dangerous situation right now.  Do you let him out in an arena to let off some steam if he's kept in a small corral or stall before you get on or round pen him a little to get some of that energy out?
 
 

First becareful.
Second make sure the tie-down is adjusted properly.  Also I have seen horses go from rearing to bucking.
Third back off on his food now.  Sounds like he is a little fueled up.  NO MORE weight builder.
Learn how to do a one rein stop so you can get control.
The best thing I have found for the sidepassing stuff is to pull them around in a tight circle and push them forward and around (thinking cutting a barrel).
 
Trainer101 needs to pipe in here.
 
I would start with the basics again in a controlled area where you are comfortable and I do agree with Dana that some work on the ground before you get on is in order to "take the fresh off" sort of.
 
 

I have the exact same thing going on with my horse...only im on the other side of the temper tantrums.  Some of it was 'rust' from easing off in the winter...(snowmaggedon made it really hard to ride!) but the vast majority was the grain.  I am now a grain nazi.  My horse is only eating low starch-high fat pelleted grains from now on.  It keeps the weight on them, gives them a nice shiny coat and most importantly no sugar buzz.  If he is on sweet feed, when it breaks down in digestion it is almost 50% sugar.  Change the feed if you can...lunge before every ride (i sometimes even lunge after) and hang in there!  Spring fever will pass!

Lol I guess I'm highly recomended!  Thanks guys.
 
Yep you guessed it groundwork, groundwork, groundwork.  This is exactly how the halter horse behaived for the first two weeks of his training.  We couldn't teach him anything until he had burned off all the excess energy and even then he really was too buzzed to learn. 
 
As the others suggested I would cut the grain down or change to something like Nutrena's Safe Choice that is easy to digest but without all the energy build up.  IF you are feeding a suplement as well check to make sure you arn't giving him too much of one thing if you do switch to a new pelleted feed as most of them have added vitamins and menerals.
 
Second I would just start out with a daily regiment of controled excersise.  If you have a round pen take him in there and take a stop watch with you, tack him all up too so that he is carring some weight.  Let him race around as hard and fast as he likes till he slows down and starts trotting.  Now click your stop watch and have him trot 5 minutes to the left, lope 5 minutes to the left and then change directions doing it all over going to the right.  If he breaks gait drive him forward, if he wants to lope keep him loping till its not his idea anymore and then have him lope his five minutes.  It doesn't sound like very long but they learn to control themselves pretty quick.  
 
After a week of this start onlining him, having him change directions offten.  Yeilding the hindquarters and then back up before his feet stop moving.  Keep him where he is having to think on where his feet are.  I like to use trees for that and it sounds silly but if he wants to jerk back before changing directions or if he trys to beat you so that you can't step out infront of him to change the direction.  Example if you are lounging him left and step to the left to have him face you and turn but he just runs sideways from you tring to evade the change, the trees will take care of that problem.  Ask him to lounge to the left or right and let the tree come between you and him.  Now you have some leverage on the lead and if he ignores it he will wind against the tree bumping his nose at the same time you can be there to send him off in the other direction.  Don't let him stop though, keep his feet moving and have him hustle so that he has to reall come off of that halter and after a while he will start coming off of the feel of the halter and looking for you for direction instead of just running the circle.
 
Been wanting to do an article on this anyway so I might just have to come up with some pics of this.  I hope this gives you some ideas but the best thing for him is some constructive excersise.  Give him obsticals to overcome and something to focus his mind on after the built up energy of the feed wears off.  You'll know when he is ready to ride again and even then he most likely will test you.  Thats why you do this on the ground and then again when you get in the saddle.  Have him circle trees at the trot, yeild his hindquarters and then back up.  It all translates to undersaddle so the more things you can think of to through at him on the ground will help you undersaddle because it will be something that he understands what the right answer should be already.
 
Good luck and stay safe!  Keep us posted.
 
 

Oh forgot to mention.  Take the tiedown off before you start doing these excersises.  The rearing is a whole other animal so work on getting him soft on the ground and that should help with the rearing.  Chances are the rearing can be fixed with some hindquarter work and teaching him to soften to the bit.  Does he nose out and push the bit when you ride?  If you pick up on the right rein does he keep moving to the left?  If so the online work will greatly improve this, but then it will be up to you to teach him how to yeild to bit pressure with lateral flexion and bending him to a stop from all gaits.

corinowalk: What feed do you use?  I use ADM SeniorGlo.