One mis-step you may have made is to push your toe too far into the stirrup and poke him in the side and startle him. Many new riders do this inadvertently. One thing that may help you is to use a mounting block to get on until you are more confident. This will help you get on easier as well as make it easier in your horse as you get more proficient at mounting. You may want to take some basic riding lessons so you two will get along well. If he is not bucking for your boyfriend at all, it may just be that your horse is not real steady for a new rider. The previous owners were of course used to him and he was confident with them. If he is bucking with your boyfriend too, he may be painful somewhere so you should have your vet check him out including a chiropractic exam. Or he may not be as well trained as you thought. Five years old is still fairly young and he may still be a little green. Some unscrupulous people trying to sell a horse will give them a mild sedative to help control a less-than-well-trained horse to make them appear quiet and rideable while prospective owners try them out. Then when they get to their new home, they show their true colors after the drugs wear off.
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