You can start them on hay at any size. Even babies if you take the time to cut it up super fine, soak it and mix it with other greens. I usually start them on hay at about 12". I just start feeding them their other foods on a bed of either orchard grass hay or Bermuda grass hay. After I while, I'll start sprinkling a little hay on top of the other foods. They take to it quickly and easily this way and begin eating the hay by itself within a few weeks.Also is he big enough to start eating these?