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He will be struggling to balance himself with your weight on top, this scares him and he takes refuge in bucking.
I would continue to do plenty of ridden work in walk and trot (also sitting trot) so that his overall balance with a rider on top improves. When working in trot increase and decrease impulsion.
Also lunge him in hand and include canter with this. You could lunge him with some weight (such as a big bags filled with straw or shavings) attached safely to his saddle. Obviously you have to start with something light and small and then work with something larger and heavier once he's comfortable with it.
Ride him out with an experienced horse in front and include short canter periods. If he follows the horse in front in a manic trot, that's ok for now, don't force him to canter yet. Try sitting in a light seat.
Work over short uphill and downhill areas is a good idea, try asking him to go uphill over short distances where he HAS to canter, even if he only does a few strides.
You could also have a look at some natural horsemanship games (Parelli) that you can do with him to increase his trust in you.
Don't over-face him, make sure he enjoys his work, it might take some time but he will get there eventually.
I assume you have had his saddle checked for fit and that there are no physical problems that would want him to buck.
Hope this helps.
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- thank you :)