Yes, stabled horses sometimes become excited when exercised and will buck. Other reasons are ill-fitting tack and sheer naughtiness. Firstly, rule out ill-fitting tack. If the saddle pinches when he gets his engine moving then he will buck and it's not his fault - get it sorted. If tack is not the problem then you must let him know his behaviour is unacceptable. Give him a tap with your whip the instant he bucks - and be prepared for another buck. He will eventually get the message that he will be smacked if he bucks and will stop - be persistent but take care to school him as if he is in a menege there is little scope for him to bolt out of control whereas if he is in a field then he can cart you off when he is reprimanded. He will not like it but he will understand it and should avoid the behaviour that results in a smack!