To increase his winrate (atleast jungle) he needs something which let him gank early. Right now there are 2 options to gank early 1: Walk straight into the lane and hope the enemy isn't walking away. 2: You are very lucky and the enemy wants to ward the river bush you are about to jump on him. As both thing are very unlikly the only other thing rengar can do then is contest the enemy jungler in his jungle, unlucky for rengar is that all the meta junglers are stronger then him at this time of the game. So he is left with just farming and waiting for level 6.