do not reasure!!! ever! You're basically praising the bad behaviour when your doing that. mother dogs nudge their pups forward when they are struggling socially. so just stand behind him and gently nudge forward, have the male in question carry something amazing treat he never gets normally. like Cheeze wiz or something really yummy, and everytime he sniffs the man, he gets a teeth, after a while he'll want to get closer and maybe get petted just work slowly, encourage ALL sniffing, it signals curiousity, and make sure your volunteer moves slowly at first. Overtime the problem should go away and people will just be people.