He falls off the second people realize you can blow his ass up... and he starts having to build tank. Literally a Morg Q into a Cait trap and he'll explode if he isn't building tank. He's super easy to kite, isn't all that tanky if he's building any damage items, and only does solid damage if you group up inside his Q range.
To counter Renekton you don't get tankier.. you just build more damage and kill him. Just like a Yi, Xin, Firoa, ect. Dead crocs aren't very scary. Land a CC and watch him explode. If your team can't kill him inside 2 seconds if he is snowballing with damage items... it isn't the Renekton that's the problem. Fiora/Riven would have already gotten gold from your nexus.
At the end of the day Renekton is just a worse Riven. Treat Renektons R like Rivens E and play around it. Except Renekton's gonna be out of it a LOT longer than Riven will. Renekton is less mobile, less CC, less burst, WAY less dps, crap escapes, and generally only strong in lane vs certain champs.
If Renekton is building tanky... odds are the game is already won. If he's snowballing that probably mean both mid AND jungle lost as well or your team made a really dumb dragon call when the 3/0 Renekton had TP up. Any game is likely lost with any top laner if only your bot lane is doing well.. unless you have Twitch/Vayne.... cause adc 2017 yo. Farm, don't die for objectives you won't get, and eventually he's either blown up by the ADC, or starts having to get tanky/fall off. Renekton's base damage values are actually pretty crappy so he becomes a non-issue.