Well, if you haven't already, you should quarantine him for at least three weeks. May seem pointless since they have already had contact, but better safe than sorry!
Then, if you are able, put him in a separate enclosure (like a dog crate) inside your coop/run where he and your chickens can get to know each other without being able to get at each other. Try to keep him separated like that for a couple of weeks and then try to introduce them again. There will be some pecking while they all get the new pecking order down. Expect it. As long as it doesn't get too bloody, let them be to get it out of their systems.
Good luck.
