I would probably start at a less amount and work up to the full cup by the time he turns 6 months as I was giving the full amount to my 6 month olds, once I had gotten them used to a variety, but introduce any new veggies slowly to avoid an upset tummy. Most of my pigs were already around 6 months when they were rescued, so I don't have a lot of experienced with younger babies.
Is he on alfalfa or timothy based pellets? if he is on alfalfa based, he should be switched to timothy based at 6 months, if he is on timothy based, he can have some alfalfa hay as he is still growing and needs the extra calcium alfalfa hay has.
I hope this helps