My horse will not let me mount!?

My horse won't let me mount! He is very stiff and he used to be in a riding school. He is 15. When I bring him to the mounting block he swivvels around and when I finally get him to stay still when I put my foot in the stirrup he starts walking away! There is not anything wrong with him, he is a quiet ride besides the stiffness. He completely stops walking when I get about a meter away from the mounting block and I have to drag him. What I find works to get him to the mounting block his standing so my back if facing him and put my left hand out and hit him lightly on the shoulder. And he walks forward but then he still swivvels away, :'(