Keeping it short and sweet: Volibear has long cooldowns but they are powerful so working on CD reduction is vital. He gains damage on his Frenzy (Main ability move) Dependent on his health. So stacking health is also increasing your damage.

It is obvious that this build lacks magic resistance and this is because it is focusing on damage. Try swapping to Mercury Treads for more Magic Resistance if it is required. IMO Attack Speed does not really help Volibear because his abilities are not dependent on it. He needs health to power up frenzy and AP to power up Thunderclaws. If you would rather get Rylai's Crystal Scepter in place of Frozen mallet it is another item that works well in this build. The slow from the mallet is extremely useful thoe, but you could argue with Rolling Thunder and Majestic Roar enemies aren't escaping you anyway.