Your biggest problem is that you are landing in the back of the saddle when you ride, thus making it impossible for him to come off the forehand and soften his back. You want to rock back so you're sitting on your back pockets - on your seat bones - and in the centre of the saddle.
When you can sit back on your seat bones, you'll have a much more efficient driving aid, AND stopping aid! If he bucked/shied while you ride like that, chances are you'll end up over his shoulder.
Once you're sitting back comfortably, concentrate on raising your chest. Imagine you are wearing a bra with a frilly bow in the middle. Lift that bow a few inches ;)
Sitting back on your seat bones and becoming more upright in your upper body, should help you to bring your lower legs back under your body. At the moment, you are gripping with your knees, thus swinging your lower legs back. This is not a good position to be in, at it is just SO easy to come off, not only that but you are also putting your horse on the forehand by putting all your weight over his front legs, and giving ineffective leg aids as your legs are too far back.
Your elbows need attention as well. At the moment, with you elbows straight and hands resting on his wither, you are unable to 'feel' his mouth and 'walk your hands' with him, thus he will be copping a good clunk in the mouth at every stride in walk and canter. Bring your elbows back, imagining that they are nailed to your hips. You could also imagine that you have weights tied to your elbows, pulling them down ;) The elbows should be the heaviest part of our arm.
When you're elbows are back against your body, you will have far more ability to move your hands in time with your horse's movement - he will thank you for it!
Carry your hands by yourself, your horse is already carrying your weight, he does not need to hold your hands up on his neck as well ;) That's YOUR job!
Hope that helps, he's a very cute little guy and there are no issues with his weight. He's just undermuscled, but that cannot be improved under saddle under your position has become effective so that you are able to help him stretch into the bridle, soften his back and engage his hind legs, which will build topline on him :)