Have you followed the following sequences?
Imprisoned in Pewter Jail, George can be found incarcerated in the fourth cell found moving eastwards.
In order to pass through the encaging door for the first time, you must first speak to Warden Noisia who is marshaled nearby; if you have completed the Legendary Beasts quest, he will assent your entry to his cell on your pretext of an "urgent" visitation you want to make to him. After the initial visit, you do not need to entreaty Noisia's permission again.
Contingent that you have completed the Kanto League, a battle with George will be initiable upon interaction with him.
He will be rechallengable after a 12-day-cooldown time has elapsed irrespective of the outcome; before then, he can still be interacted with, making a random pleasantry each time before offering to quantify his remaining cooldown time.