How to fix a bucking problem?

My gelding is young(4) and bucks when he gallops or when i let him have his head when cantering or loping. He is a great horse he just gets spunky. Last year he threw my sis off and twisted 2 vertebres around in her neck, he was in training for 2 months after that and stopped bucking but now he started doing it again and i cant afford to send him back to training and im only 15 so i dont know how to fix it myself yet.. He doesnt buck hard with me b/c he knows he cant get away with it though, he just does little hopping bucks so he has never gotten me off him but i dont want to give him a chance to either. Every time he does it i pop him on the neck, back him up then try again and repeat that untill he stops bucking. how can i get it to stay in his head without having to do that every single time he gallops?