Donāt even tell him. It was only a joke. Callum needs to stop worrying about where he can hoax people about time travel and seek serious psychiatric help. If you are his friend, maybe try to get him to see a counsellor/psychiatrist to figure out his issues. Itās not normal to be as fixated on this many fantasies and Iād be pretty concerned if that was my friend.
Especially if he truly believes these 'Fantasies' are reality.