When was the first time Jason was unmasked

Part 2, I think.

possibly part 2. but he definately takes his mask off in part 3 for 1 scene

Part 2. He actually appears unmasked in all of the first 8 films (as a child in Part I). I think this film is the LONGEST he goes unmasked though.

Out of curiosity, which ones have you not seen?

He appears as a child unmasked in a dream sequence in the first film.
He wears a sack in the second film and is unmasked briefly during a battle.
In the third film he finally gets the hockey mask which he loses briefly during the final battle.
So depending on your definition of "unmasked" one of those three.