If you can get something spray on it might help otherwise don't focus on putting the cream on but instead do other things with him When I get a horse like this (and for some reason TB's seem to be bigger babies than any others) I start out with grooming all over paying extra attention to the legs but not spending too much time on each one - just repetition and then moving off onto another part of the body - I find it helps if someone else is also messing with them, combing the mane, brushing the tail, face etc so the horse stops thinking about the leg and relaxes then when brushing around the injured area I sneakily slap some cream on
Never let him think you are just going in to treat his wounds as some horses can get really paranoid and defensive.