If i casr whirlwind can i force opponent to bounce his baron nashor

Yes — Baron Nashor is a perfectly legal choice for Whirlwind, but you can't force it; the opponent makes their own choice during resolution.

Whirlwind never targets or "chooses" units — it just says each player may return a unit to its owner's hand, with players making that choice as Whirlwind resolves. Since the spell itself isn't choosing Baron, his "I can't be chosen by enemy spells and abilities" protection doesn't apply (FAQ #10445). So the opponent may pick Baron Nashor and bounce him — but they could also pick one of their other units instead, or decline if no other legal target exists. You cannot make them choose Baron specifically.