I cannot seem to be able to rehire Arcade Gannon from the Mormon Fort. If I approach him with another human follower, he gives the standard "three's a small army" response.

If I approach him with no followers at all, he says that I have "quite the menagerie" at my disposal even though no one else is following me.

Talking to him with just Rex or ED-E makes it so there's no dialogue choices about hiring him.

Someone help please? I really want to do For Auld Lang Sine.

Sounds like a bug, try dismissing all your companions. If your on PC you can fix this. If not, well...

Erm,m bad rep with the Followers or Freeside maybe ?

No, I'm Liked with them. I'm Vilified with the Legion, so I know that isn't the problem either.