With any new foods, you should introduce it to the rabbit's diet slowly, a bit at a time. Give the rabbit a small piece and see what he does. It's not uncommon for a rabbit to react cautiously at first and reject a new food, then the next day eat it like it's going out of style - after all, he remembers seeing it before at that point.
At three months old you can start introducing leafy vegetables, but you need to be particularly careful to watch the bunny and make sure that there's no adverse reaction. Go slowly, introduce one new food at a time, and see what happens.