If you haven’t tried this already, maybe spread his front feet at the block (gently rock his front end back and forth), so that he is balanced but can’t go forward as easily, and take up the outside rein a bit so that when/if he moves, he moves into you as opposed to away. Might want to look into a taller mounting block, too : ) Also, check saddle fit, bridle fit (includes bit, cavesson and dental eval) and consider the riding skills he is under most often. Horses have their reasons for everything they do. The joy, challenge and reward of horsemanship (for me) is figuring out what those reasons are. Good luck and be safe!!
Sure there's right and wrong but mostly there's just a whole lotta different.