The issue here is that while he respects you when JUST with you, he doesn't respect your leadership AROUND OTHER HORSES. He still takes on that Alpha role, but when you step into that paddock YOU are Alpha of the herd and no to little dominance games should go on. That's how it is with my horses, when I enter the picture all that stuff stops. That's how it should be :) So the best thing to do is to take a lead rope/training stick (not a lunge whip b/c that can sting) and when he starts acting aggressive toward other horses drive him away assertively and do not let him come back until he has a nice look on his face. Invite him back and ignore him, but drive him away again if he starts acting aggressively. You are basically taking on the role of lead mare in the herd, driving the snotty troublemaker away from the rest of the herd until he changes his mind.