Free choice grass hay, mixed grass hay, or timothy hay, all he can eat.
LOTS of turn out time, preferably 24/7. Movement is key to growing joints.
For "feed", I like Alfalfa pellets mixed with flax and a vitamin supplement. I'm not a fan of grains or pelleted feeds. Alfalfa has lots of protein for growing horses, and you can buy vitamin supplements geared toward young horses, like SmartPak's SmartVite Foal. The flax gives extra fat. If you do a smartpak, you can use Glanzen, Horseshine, or Gleam & Gain. I use NutraFlax from horsetech.com and really like it. Shipping is free from them.