Some thoughts:
His main issue is that he can't clear properly without running out of mana in an instant, if his blue buff is stolen away his clear takes AGES - it also means that he's horribly inflexible when it comes to gifting blue to his midlaner. Not to mention that it makes him VERY vulnerable to invades and bad at counterjungling as well, or following up ganks/teamfights with objectives like dragon - (since he will most likely lack the mana to do it in a timely manner).
Suggestion:
A possible change could be a mana refund when hitting monsters in order to avoid teleport solo-lane abuse like we've seen it in the past.
Why do I suggest an earlygame mana cost buff?:
Other changes, like adding durability would be more of a lategame focused changes, while he already scales relatively well into lategame due to strong item synergies (Triforce, Dead Man's Plate, Black Cleaver).
However: his issues are rather based on his earlygame (and particularly jungle clear) being so weak that he doesn't reach the lategame without being severely behind due to his a.) lackluster clear, b.) medicore pre-6 ganks and c.) his inability to duel other junglers 1on1.