Just before speaking to Jonathan, he pointed an intense finger at Bill and told her, "I'm not granddad. Don't use that name around him, I don't want it to stick."
Outside, the Doctor stepped onto the pavement and with a gesture of his hand, invited Jonathan through the door. "Welcome to your new place!" he said with a grin.
There was nothing quite like watching a new person's first reaction to the technical marvel that was the TARDIS - the Doctor liked to think he could determine somebody's personality by how they reacted. For example, Bill... her first response was viewing the TARDIS in a completely unique and off the wall way (she looked like a kitchen? Seriously?), which matched her distinctively peculiar personality.