You know, this probably sound weird but my vet races horses and when we were given a young horse with a horrible injury (she had a flap of skin about 8" cut off!) he recommended a solution of 1 part bleach, 2 parts water and that horse stood like a rock and now you can't even see the mark on her skin unless you look realllllyyyy close! It was amazing! He said it would keep the wound clean and kill all bad bacteria.
Oh, I'm sorry. If the tendon wasn't cut and there is no infection, then don't stress over tendon death yet. I'm sure that he will heal up just fine. At the risk of sounding cold, that is just all a part of the horse business. Deaths, injuries, lameness, behavioral issues, etc. We all just have to take it one day at a time and do the best we can. Know that my thoughts are with you and Denny (I have a horse of the same name by the way).
How in the world do you miss that even if you are not paying attention?! Heck, he had to limp or something! I know even if I am not paying to much attention and bring one of my horses in, I know I would see that!