PLEASE use a ground pole! It is REALLY hard for inexperienced jumpers to judge their distance without one, you are making it very hard for him to do what you want. He may be clearing jumps, but that is the LAST consideration when jumping, really. You want him balanced, calm, forward, straight and attentive. He can't be all these things if he is confused, worried, or not getting a lot of direction/help from his rider. Basically you are tipping forward before the jump, and have no contact. This means you have lost your base of support. I'm suprised you didn't fly over his head when he stopped, you must have been going very steady. You need to SIT UP before the jump and maintain a good contact. This will support him and allow you to help him out by manipulating his stride to ensure he finds the CORRECT take-off spot. Jumping small jumps like that you don't need to crouch over his neck. Really all you need is a light 3 point/very slight 2 point and just enough rein to allow him to stretch out. Throwing your rein away before the jump will undermine his confidence, as you are leaving everything up to him and giving him no help. As I said above, you need to sit up, maintain a contact, and ride him forward and solidly into a jump with a ground pole.