Wikipedia definition of ex post facto:
... is a law that retroactively changes the legal consequences (or status) of actions that were committed, or relationships that existed, before the enactment of the law. In criminal law, it may criminalize actions that were legal when committed; it may aggravate a crime by bringing it into a more severe category than it was in when it was committed; it may change the punishment prescribed for a crime, as by adding new penalties or extending sentences; or it may alter the rules of evidence in order to make conviction for a crime likelier than it would have been when the deed was committed.
How does this not change the legal consequences, and/or aggravate the crime?
If losing his 2A rights is a legal consequence, and how can it not be...?