renekton is, and always will b, a early game champ/lane bully. he thrives of abusing mistakes to get himself ahead trough his short / uniteractive trades.
and while spear of shojin does allot to fix his horrible lategame there are ways to counter the mad croc. and consequently never allow him to reach that point.
for example you can start 
while having the +6 armor rune.
with this start you can easily stay in lane longer against the croc to CS or even apply pressure, renekton will usually take press the attack instead of conqueror due to the latter only having more impact in the lategame. (when he has 40% CDR)
other ways to counter him in-lane is by playing a champion which can force a long trade, as renekton hates these. champions such as Darius come to mind. as for his level 6 all-in that is something that you have to watch out for, if he is about to hit level 6 back off, if he already has it bait it out and then use mobility to get out of his ranges. without his ult renekton cannot do much post level 6.
so how to beat him:
-play safe, he is meant to do good early then fall off.
-build some early armor, this cuts heavily into renekton's damage
-force him into long trades, he often loses these (see: darius, mord, olaf)
-bait his ult / make him waste flash. without these he cant do much.
-gank his ass, this always works especially pre-6.
That's all
