Veterans

It is considered an administrative punishment. There is no ability to reduce rank (for an officer, enlisted may be reduced a grade) or dismiss a person from service. The closest civilian analogy would be going before a magistrate for a misdemeanor charge.

His resignation would have been part of the plea deal to avoid a Court Martial. It would still be considered voluntary as officers may choose to resign at any time (with certain limitations)

So yes he is allowed to refer to himself as LTC (ret.)