Renekton its a melee carry ad dps, it is difficult to manage him, specially is you dont make a good job in early game.

Armor Penetration: 3 of his skills have ad damage so 9 armor penetration marks would be great for him.

Dodge: In your masteries you got 2% dodge chance plus this runes would be a great combination including the boost of 10% speed everytime you dodge.
Magic Ressistance per level: You're a melee champion so you need some MR to stay alive in battle as long as you can.
Health: 3 of these quintissence would be great for the early game its good always to have more health than the other team.

9-21-0 WHY?
As youre a melee champion you will need to survive during battles and do as much damage as u can.

Doran Shield: This item with a pot would be great to stay in line and dont go back, to creep as fast as you can.
Mercury treads: I think you hate stuns, slows, taunts as me so this would be great to stand in battle and also to run from them.
Ghostblade: Armor penetration + Damage + Attack speed + Movement Speed works perfectly with Renekton.
Last Whisper: 40% Armor penetration for those tanks hard to kill
Frozen Mallet: Excellent health afford + a little damage and the possibility to keep following your enemies and slowing them :D
Impaler: You got some Health now its time for some damage and armor for survavility.
Bloodthirster: More damage and lifesteal :D