I just played a Renekton as Nasus. His early game is his strongest. He almost got me a few times but once I had a few items I could out trade him and ultimately kill him in lane. His cooldowns are long with no cdr so take advantage of that.
Another thing, when you see him ult, do NOT engage. The damage from his ultimate + a sunfire cape(if he has one will wreck squishes) also pay attention to his fury bar, I don't have exact number but when he's over 50(bar turns read) all his abilities get amplified in damage especially his W.
I've been seeing a lot of Renektons lately, and they all have been doing one thing...they build damage. But it leaves them extremely weak and can be bursted down. Try to poke him and ping for jungler. You need to try to bait out his combo, after he E-Q-W-E hes got nothing and can be killed. Try have him towards your side of river when he goes in with his E.
Good luck. If he doesnt build resistances by late game he'll fall really quick.