Hitler was baptized in a Catholic church, he never renounced Catholicism, never converted to another faith, and was never excommunicated.
Actually, under Canon Law, there's something called latae sententae excommunication, under which a person is automatically excommunicated upon commission of certain offenses, without any bishop having to declare it.
Freeper Campion, a couple years ago, pointed out that Hitler did in fact commit acts that resulted in his latae sententae excommunication:
"The first time he committed or instigated a violent act against a priest or religious (1934, for sure, but maybe before then), OR the first time he stole or intentionally destroyed church property (1939, if not earlier), OR the first time he authorized or promoted, or forced an abortion for anyone (Jews and Poles were victimized by forced abortion during the Holocaust), he was excommunicated automatically, according to canon law."
So to answer your question, under Canon Law, Hitler was not legally a Catholic from at least 1934, and probably earlier, his statements to the contrary notwithstanding.
My sister and her ex-husband have had an abortion, at his behest, yet both are permitted to attend the catholic church in their small town where everyone knows everyone else's business. That's really a heck of an excommunication. Hitler must have sure cried himself to sleep the night he heard about this big ol' hairy ex-communication.