9-21-0, Take attack speed etc.. in offence, and standard Jungle masteries in defence.

9 x Attack Speed Reds
9x Armour Yellows
9 x Scaling mr blues
3 x Lifesteal quints

"So, Jayce Jungle? I thought Jayce was a top/mid lane solo brusier/carry... thing."
Well, yes he is. However, with the recent changes to his e, he now has less of a presence throughout the game. So, I decided to innovate and try him in the jungle, and it makes perfect sense! Let me show you...

Let's have a look at his abilities...

Q) In ranged form, it is actually his least effective ability as a jungler... this won't be doing damage until very late game. However, in melee form, this provides a slow which is key to Jayce's ganking potential... Allowing him to walk past and hit them back for an E in melee form.

W) The skill you'll be maxing first. In ranged form, this does a nice bit of burst which helps to apply red buff (kinda), which is another good aspect of Jayce jungle. HE CAN APPLY RED BUFF FROM RANGE. Many people underestimate how potent this can be. In melee form, this helps to clear wraith + wolf camps within seconds by the time you have maxed it. It also, when maxed, is the only reason Jayce doesn't run out of mana in the jungle, surprisingly.

E) In ranged form, this can be applied two ways. Either to speed yourself up to apply red buff, then switch to hammer form to Q; or if you manage to close the distance without it, use it to get your laner in place! In melee form, it's the standard knockback you've always loved!

R) Ranking this up gives Jayce more survavability in his melee form, allowing him to be healthier in the jungle!

Outside of his abilities, the only other reason Jayce is a good jungler is because you get that beloved poke (whilst not as potent) in the late game, as well as the engage/disengage of his E. Also, if coupled with a vayne, then you automatically win late game - enough said.

Finally, his e+q combo can steal buffs from longe range. Have fun with that!

Well, his early game jungle is a tiny bit weak, as he often drops health quite a bit, resolved slightly by the Lifesteal Quints. Also, he's not a naturally tanky jungler, unless you heavily build him that way.
CC wise, he only has a slow, a speed up, and a knockback - which is less than most common junglers out there. Therefore he doesn't provide that "Centre of the teamfight" presence, and probably shares more of a role with Elise, a midline fighter.