The only possible hard argument I can think of is, there was no ULEZ sign at the ULEZ boundary on the road he drove into London on. To be honest, he must have been living on Tristan da Cunha not to have known about ULEZ, as it was in all the national newspapers when the zone was expanded under Sadiq Khan, the Mayor, (who got re-elected recently).
However, nothing ventured, nothing gained, so I suggest submitting representations on not having visited London for many years, living far away in a rural are, (if true), etc and therefore not knowing about the ULEZ zone.
Of course if he can show there was no ULEZ sign on the appproach route it would help.
He has the right in law, if TfL declines his reps, to take them to London Tribunals, but this is with the full PCN penalty in play.
Edit
Please also note that adjudicators at London Tribunals cannot consider appeals based on mitigation.