Have just read your other thread. I'd be really interested to find out what it is. Obviously hope it's nothing serious and that he gets better very soon. I'm sure this doesn't help in the slightest but I shall put it anyway. Horses get these, in a very similar place to where you say Shadow's is, they're called splints. They're bony growths that usually young horses get, quite often from overworking on hard surfaces. Quite often they are tender but after a week-three weeks rest the horse is usually fine and ready to go back into work. They always have the lump but normally no more pain in it. I know dogs and horses are very different and I know an awful lot more about horses than dogs, but maybe it is something like that?
Anyway, I hope it's nothing too much to worry about and that he is back to his usual Cocker mischief in no time!