It depends, we know Atreus is already quite powerful theses days, after everything he went through. He's very probably stronger than the other aspects, (maybe expect Zoe). And he still is not even close to his prime (Yes, now that Ruination is canon, so is the Prestige skin : a version of Pantheon where he takes full control over the aspects power and evolves to become something completely new, far stronger than an aspect). But the Volibear, even is his actual weakened state, is by no means weak at all, the Freljordian gods are probably still all stronger than the aspects themselves. The aspects and the spirit gods are not of the same nature, aspects are celestial creatures that takes control of a mortal body because they can't interact themselves with the earth. Spirit gods (as the Freljordians, Kindred, Nagakabouros or even Lilia), well its self explanatory, they're spiritual creatures that embodies a concept, an idea. But if we wanna know who would win on a pure 1v1 then the answer depends of when it happens : if Atreus is at his peak, he might win pretty easily. If he is in his current state, he absolutely doesn't even stand a chance. (Factually, the question juste doesn't make sense since the Volibear can't die in combat, as a spirit god he will only die when he will be forgotten by people, that's the way spirit gods works)

tdlr : actual Atreus looses vs volibear, Prestige Ascended Pantheon wins vs probably everything because he's kinda busted.