does backoff work on baron

No. Back Off can't stun Baron Nashor.

Back Off says "[Stun] a unit," which requires you to choose a unit — that makes it a targeted effect (Rules 355.10, 423.1). Baron's protection — "I can't be chosen by enemy spells and abilities" — makes him an illegal choice for any enemy spell or ability that targets, including Back Off (FAQ #10502).

One nuance: if you control both the Back Off and Baron (or are otherwise not its enemy), his protection doesn't apply and he can be stunned normally. But from an opponent, Back Off can't pick him at all.