When you are cantering try and keep his head up, when you feel him start to lower it, sit back and squeeze him and keep a steady hold on his mouth (don't yank on him) just keep a consistent but firm hold and raise your hands up. Exaggerate it, it will feel really weird, but the higher your hands the harder it is for him to lower his head. It also makes him shorten his stride. If his head is up its harder for him to buck. Also if he does buck, you'll already be sitting back, so it's less likely you'll fall off. Practice that when you are schooling in the arena, so you can get the feel of it before you go out on the grassy areas.