Heartsteel
Heartsteel is great, as usual. Rengar does no %HP damage, and he's pretty squishy so you can pop him with your passive and Heartsteel's passive, of which both are great against squishies. The range is also nice.
Swiftness Boots
Swifties are nice, as they allow you to chase him into bushes really easy, and bully him. I think tabi's is a total bait, in general, but especially here as Rengar's damage isn't really all that scary to you as much as it is to a squishy champion.
Doran's Shield
I'd like to note that Doran's Shield has a lot of value here early on. If he ever jumps onto you, you can just regenerate a portion of it with D-Shield. If you don't default Shield, I'd at least take it here.