I agree with equinesalways, it looks like something is possibly hurting him, which could also lead to the head tossing. As for your position, raise your stirrups a few holes, the bar of the stirrup should hit the ball of your ankle. Bring your lower leg back alittle, you want your ankle, hip & ear in a line. You look like your posting from the back of your saddle, stay in the front of your saddle & you only need to post as big as he pushes you out of the saddle. Also at the working trot, bend about 20 degrees forward with your hip so you post over him. Bend your elbows more, you want softness from your elbow by opening & closing it, but you also want a straight line from your elbow to the bit. At the canter, soften up your back & hips slightly & follow his mouth with your hands. Although your position could use some work(like everyone's could, nobody's perfect!) it is a pretty good position for not having a trainer & it looks like you've made some improvement from a year ago!