Alright, first off don't judge me too much on this build because I haven't gotten to play Volibear much but I definetly think he has a lot of potential. I find Volibear as highly versatile and many builds can work. This build is very flexible and I'll go in depth to different items you can get depending on your team and the other team.

I try to keep about 12 armor pen because most jungle creeps have about 12 armor. Then I take some attack speed because volibear has his W and Ulti that benefit from attack speed. Flat armor yellows for easier jungle and then CD reduction blues or Mr per lvl. They're both good and their up to you on which you would rather do.

This is an interesting mastery set up but I think it works pretty well. I got a little bit of offense, mostly defense and then some utility to help my jungle and ganking. The reason I spec 9 into offensive masteries is to get the extra damage to minions, 35% speed buff for ghost and then the extra attack speed for jungling and ganking. Defense helps with survivability and gives more damage on the activation of Frenzy. Also the extra hp regen per 5 is good for the jungle. The utility specs give a 2% movement speed buff and then the 20% longer buffs from the jungle is great.

I'll say it again, this build is very flexible. Now I will go into detail about all the items that I feel are viable.
Boots:
Mercury Treads
Ninja Tabi
Maybe even boots of swiftness if your feelin' lucky

Offensive items:
Atma's Impaler (depending on hp you have)
Madred's Bloodrazor
Wit's End
Wriggles Lantern
Frozen Mallet

Defensive items:
Frozen Heart
Warmog's Armor
Banshee's Veil
Force of Nature
Rod of Ages (also an offensive item because of AP)
Spirit Visage
Randuin's Omen
Aegis of the Legion

This build definetly varies on what your going up against. Clearly if your going against a heavy AD team get ninja tabai and frozen heart or randuins and if your going up against a heavy AP team get merc treads and force of nature. I like the wits end to speed up my jungle path and also for doing more damage with the ultimate Stormbringer.

Frenzy is definitely Volibear's strongest spell. With really high health you dish out a lot of damage with the activation bite. I max this first for this specific purpose, more damage. I max Thundering Smash next because of the speed buff, the damage it does and its fantastic use for ganking. I only take one rank of Sky Splitter early to help take less damage from minion camps because of the long fear and small damage it does.

Well since this is a jungle guide then the must have is smite.
Here's a list of viable summoner spells in my opinion:
Ghost
Heal
[Exhaust]]
Flash (not very good though)

If you plan on doing Heal + Smite then I recommend getting a Spirit Visage.

For the jungle route I start out with cloth armor and 5 hp pots and head of to double golems. Smite the big one and kill the camp. Go to wraiths, then wolves, then blue and use smite again. Recall back and then start at golems again, this time get red after golems so save your smite for the red buff. This is a chance to gank because you'll be lvl 3 and have your Q Thundering Smash, W Frenzy , E Sky Splitter (everything you need for a gank). Now that the jungle is very weird is is hard to keep up with the laners with volibear because hes not too fast of a jungler, especially mid to late game when he becomes more tank based. Try and cover lanes and get a lot of ganks to keep up with everyone.