Do you see the picture where you're left behind? You honestly barely need to be any further out of the saddle than that. For jumps this size, you don't need to be far out of the saddle or down low on his neck at all. Instead of jumping for him, which is esentially what people are doing when they stand up over jumps, try to jump more with him by just folding forward a bit and allowing the momentum of the jump to bring you slightly out of the saddle. An exercise you could try would be to hold the reins backwards (just grab them from the top in a fist) & go over a few jumps that way. It'll not only force you to give a correct release, but it'll make you really feel him jumping.