How to stop a horse from bucking when I get him into a lope

As has already been suggested, rule out pain or tack fit first.

Think of it this way. You're training for football. And you've got a broken hand. Sure, you can run and down the field no problem pain free, but when you try to throw, or catch yourself doing up-downs, it hurts. Now imagine your coach chewing you out because your throws stink, and yelling at you to hurry up and do your up-downs correctly. It's not that your not trained or not trying. It's that is HURTS!

That's why it is always a good idea to rule out pain or tack fit when working with an unfamiliar horse or with an "attitude" problem that seems to arise out of the blue.

If you discover that the horse is not in pain, then you can work on the behavior and fixing the attitude. If the horse doesn't want to gallop, make sure you MAKE them gallop. By bucking, they are trying to get out of work. If you allow them to get out of work, guess what? The horse "wins" and gets what they want. Do circles, figure 8's, and whatever at the gallop and do not stop until YOU ask the horse to stop (not when the horse decides to throw a bucking fit). You don't have to be abusive or spur excessively or any of that. But you do need to be firm and consistent every single time.

To add on to this, make sure you only set yourself up for a fight you can win. If I'm having a really hard time getting a horse to canter, I won't make than canter and canter and canter because if they break stride, it's just going to be another big fight to get them to pick it up again. You want to push them further than they want to go, but stop before they start a fight!
Posted via Mobile Device

As he's not happy trotting either he either has a possible back pain issue or just hates work in general - could be plain idleness or boredom or some other health issue that makes him resent work
What is he like when he's lunged or worked in a round pen without a rider?
Are you paying to ride this horse in a lesson situation? If you are you should refuse to ride him until the owner has sorted out the problem

My pony used to "crowhop" at anything faster than a walk and it turned out the issue was with the saddle (knot that was poking his side) and the bit (too small curb). Once we fixed those, he still needed work because he would anticipate the pain and start crowhopping as soon as we asked for a trot or a canter. Once he figured out it wasn't going to hurt, he got the hang of it and stopped bucking.

You have gotten good suggestions. If it is not a pain issue, or once that is fixed, I, too, like to bring a horse back down to a trot. A strong, working trot for a while before asking for a lope again. Most often that works.

I have, however, run into a few that will try to buck again and again. It seemed to be their way of getting a rider to give up and stop the ride (work). On those few, I made them gallop. A long time. When they wanted to slow to a lope I urged them forward, only transitioning to a lope when I asked. After a short lope, we walk. After they have caught their breath I ask for a lope.

Usually in three, and a couple of times it took five sessions, they would lope off nicely enough. I had to use more leg to urge them forward when they thought of testing me, but this behavior passed soon enough.

I'm not averse to horses and humans sweating, though. And if you have to go this route, they (and likely you) will break a sweat.

As said before, definitely rule out pain first!

But I'd like to offer an alternative solution if pain ends up not being the issue. I use a one rein stop and disengage his hindquarters one full circle, then immediately go back to asking for the work I want them to do. The key is to really hustle their feet when you disengage. If you let them go too slow, the horse gets to rest every time they buck which is counter productive. Now I'm not saying rip their head around and spur the living heck out of them-the horse needs to be taught this at a halt FIRST, then at a walk, and so on. When you disengage the hindquarters, they can't buck and it's usually a lot more work than just doing what you asked for in the first place! Hope this helps!

Consistancy. Repetition. Ride it out. He bucks to tell you he dosent like it, but not following through with the command to lope and stopping him from going into the lope due to his behavior is only teaching him that if he does buck he dosent have to work. Horses learn from release on the pressure it self. Does he respect you on the ground? Does he give you any signs before the buck like a tail swoosh or pinning of the ears?