Overall you don't want AP items, hybrid at best, and mostly AD. His strength comes from his AA itself (passive) and his ult's passive (goes hand in hand with his passive). His W allow to reset his AA, making both passives even more deadly, the damage of the W itself is only boosted through the "on the side" AP, it's not something you want to primarily gear for.
So many ways to build him depending on situations though, all of them will allow you to solo baron around lv 15/16 but that isn't really something you want to aim for as it's either highly unpractical in a real match (ward coverage) or down right not efficient (it takes long enough for it to become not worth it if you have to leave your team 4v5).
Check
here for guides showing the core items he needs. They all have different final builds but you can easily see his core possibilities (Tabis/Merc, Triforce/Iceborn gauntlets, BOTRK, Guinsoo, Maw).
Keep in mind that all of this is expensive and hard to achieve on a jungler budget. This is why he's mainly played top, you need the money.