Horse lame. no heat or swelling. possible bruise?

the problem could be from a great number of things. You seem to be doing all you can so time is needed to allow your horse to heal. As long as he is comfortable and seems to be improving, there is nothing else to be done except to watch and wait. He could have injured his shoulder or any number of things. When you do not have an overt group of symptoms, it can be difficult or impossible to understand the problem. Horses are just accident prone. Hope he continues to improve.