i am guessing that you just grazed the top of his back, have seen and heard of deer dropping after a graze on the top of the back and they will get up and are fine. I had a doe do this that i shot in a clover field years back, walked up after the shot and she jumped up, two followup shots put her down for good. upon inspection my first shot seemingly just stunned her and dropped her as i found that i shot her on the top of her back. sounds to me like this was the case, but don't give up!