It would take quite a bit of convincing, as the lizardmen, if told of the dragon's death, would be out for blood. His best bet would be to tell each camp that they've been chosen for a special mission, to go to a different location in the swamp, and patrol for trouble for a week. This would ensure that, by the time they returned, the hobgoblins would be gone, as would the heroes, and the lizardmen, lacking organization, would disperse, although this would take the better part of two hours, though he could seriously reduce that time by teleporting between villages.