Fascism is quite different from the paternalistic reactionary conservatism Petain espoused. For one thing, fascism has a bizarre sort of egalitarianism running through it - that is to say, "all Germans are equally better than the non-aryans, but no German is better than another German" - whereas the reactionary politics of the time was very much influenced by the aristocracies and monarchies that had (or still did) ruled their countries.
So would Petain be a fascist? No. He worked with fascists, like many other conservatives and reactionaries in Europe, in the hopes of eliminating communism, but fascism is a 20th century ideology, and Petain was firmly 19th century.