It's hard to buff Volibear without changing his kit.
He has good stats and high values on his abilities. His problem is that he's incredibly predictable and easy to counterpick/counterplay.
He's still very snowbally, and I've seen him do well when he's supported by champions like Lulu or Zil. But against a lot of teams, he simply does not have the tools to reach the enemy backline. And, unlike most other juggernauts, he can't threaten frontliners.
He's an old-school bruiser. Before Riot started giving bruisers lots of %health and true damage, they would just give them massive flat base damage on their abilities. So, if he's ahead on levels, he deals a ton of damage while building pure tank, and rips apart squishies. But his damage falls off super hard late game, and can never really threaten other champions that stack tank items.
Giving him a little bit of %health damage (maybe as a passive on Frenzy) would give him something to do when he can't get on top of the enemy backline. It's unrealistic to expect him to be able to squishies; front-liners should have to care about his damage.