Some people wait till 12 weeks before introducing veggies/treats, but as long as he is eating lot of hay to keep his gut moving, I don't see why you can't start him on tiny amounts right now.
Good treats would be herbs like cilantro, basil, mint, or very small bits of banana, apple or carrot. Be sure to introduce them slowly, starting with the tiniest pieces, and keep an eye on his poops before giving him larger portions.