A cure for bucking?

That takes an awful lot of energy!  17?  Either something's wrong or he's just feeling good and realizing ya'll can't ride! [] 
JUST KIDDING!!!!
My first reaction was "beat the snot out of him and ride it out!"  Actually, when they buck it adds a little fun to the monotony of ring work. []  Not if people are getting thrown, though.  Seriously, you might double check tack fit and have a chiro check his back out.  Is he on Sr. Feed and/or joint supplements.  He might be in some kind of pain.

Without fully knowing your situation, I can only give you some options and you will have to determine which, if any, may work best for you.
1)  Lounge your gelding before riding him to get rid of excess energy.  A trot is the best gait for tiring a horse out since it uses more muscle than a walk or canter.  Additionally, if your are not worried about riding him in a trot, by all means get him tired before urging him into a canter.
2)  If the horse bucks from a slow speed, turn him in a small circle.  It is difficult if not almost impossible for a horse to maintain balance in a small circle and buck at the same time.  Of course, this option requires you to either feel his muscles start to gather for the buck and circle him before it comes or to stay on long enough if he starts bucking to turn him.
3)  A buck smooths out the faster you go, so, if at a canter, your gelding starts bucking, give him a kick and urge him to pick up the pace into a gallop.  Not only does it get smoother, but often they will eventually learn that bucking brings about more work and harder work and decide that it is not worth it.
4)  Of course there is always the option of having a more experienced rider ride him, however, I've noticed more often than not, horses know who they can intimidate and who they can not.

This is if his bucking is a behavior problem.  As the previous writer stated, it may be a back or stifle problem.  Or a tack issue.  Either way, wear a helmet and be safe.

I 'third' the suggestion to get him & his tack checked out & treated where necessary. Bucking is generally from either pain or fear. You don't say if he does it without a rider/saddle or when you last had his saddle & back checked. It's a good idea to assess the comfort of a saddle at least 6-monthly, as horses change shape with the seasons, and saddles also change over time. Especially if he only does it going into the cantEr, this is a possible indication of saddle probs, as if the saddle fits him well while standing still, this doesn't mean it's going to necessarily fit when he bunches his muscles & raises his back(or tries to) when cantering. If you don't understand saddle fit, Balance International have a heap of good info on their site.

This could also be from the rider, bit or teeth or all.  If you can have someone video tape you asking for the lope but if you can't its still simple to work through.  If you have a round pen or arena no larger than 60 ft, tack him up including the bit and take off the reins.  Free lounge him at the trot for a while till he's warmed up and not going to hurt himself then ask for the lope.  Really watch his head and body.  That should tell you where the problem is.  If its been a while since his teeth were floated have them done because when a horse lopes their lower jaw has to be able to slide comfortably back and forth across the top teeth.  If this can't happen then it will cause him pain to lope but not to trot.
 
If it isn't teeth then it could be that unknowingly when you ask for the lope you pick up on the reins, maybe to slow him down or to steady yourself.  Letting him stride out in the round pen like this without a rider but carring the bit will help him realize that each time you ask for a lope you arn't snatching the bit.  Just let him stride out and if he bucks drive him forward and make him hustle till he is loping smoothly then stop him and let him catch his breath a bit and then do it again.  Chances are it might take him a few times like this to realize that bucking isn't the answer and that it only means more work where as when you ask for a lope and he does it nice and calm let him stop and really praise him.
 
If it is this problem then instead of just hoping on and going have someone help you on the ground.  You get on after warming him up this way and keep your hands down low with slakc in the reins.  Be a pasinger so to speak and let the ground man drive him forward for you asking for the canter that way so that he can work into it again.  After a few days wean him off of the ground man and start really working on transitions.  When he lopes off smooth and calm let him stride out a while and then stop him and give him a good pat.