It might indeed by jealousy your horse is feeling, Kandylou. However, the more prudent (though far less endearing!) answer probably is that he views you as a resource, and is protecting that resource from the rest of the herd.
If you’re interested in correcting this behavior, I suggest that you do two things: leave the pasture whenever he shows aggression toward other horses, and never feed him treats while in the pasture with him. Good luck!