Havent played against him at all yet, but would like to know some of his counters if I comes up against him in ranked, thx for helping =)
Or just his weakness is fine too

I have played versus him, to be honest he will never have a hard counter the way he works now, it's just impossible . Imagine if nidalee culd slow you and knock you back, yeah...

I countered him with Akali

i beat him wiht sustain (rush life steal)

any gap closer I guess.

yorick i bet or shen...

he have GTFO hammer smash for such tricks : ) and speed boost
Havent played agains them yet, Not sure about Yoirc but his constant slow is kinda strong so yeah maybe, Shens taunt surerly prevent Jayce from guarding with E for a taunt duration so yeah, thats at least somethign what
well timed stun from jax (properly used a bit earlier) too, fast Irelia stun right after jump may work but than it is a contest who will use skill farst.
Mentioned Akali can do the trick couse she had 3 jumps from ulti and Jayce have only one E strike but still If you will time it nice to knock back her and use a fast R change + speed gate you can run out from her range.

Enemy team counters him. Twice have I had him on my team, and both of them fed hard, even the enemy Jayce. Meh, I don't know yet.

I played 2 times vs him today. people seem to tink that his 500 range makes him an ad range carry. Big mistake. Can be easily harrased by any carry. Only thing to look out is that cannon shot.

I've seen Jarvan do very well with him since Jarvcan's Level 2 is stronger then his.