youngster. naughty!!! dont dont what to do?

[RACHEL]

my gelding who is soon 4 was a sweet lovely boy who did everything a asked then suddenly he started moving and wouldn't let me on, this just got worse. Hes now started doing small bucks and refusing to do as i ask him. Iv gone back to ground work him him but the problem is when i react to his bad behaviour it makes him worse and i cant handle him but i need to show when hes bad so he cant get away with it! whats the best thing to do? im now a bit nervous and hes picking up on it