Yes you can start with leafy greens now. But start very slow. Pick one green such as romaine lettuce or bok choy and offer a small piece the size of your finger, the next day if there’s no been issues with poop slightly increase the size of the piece. Build up over the course of a week to a bigger piece. If after a week there are no tummy or poop issues then you can start introducing the next green
If at any point there are tummy or poop issues, stop feeding the green you were introducing for a 1-2 weeks and then try again. Some rabbits have trouble with greens at a young age if their mum wasn’t eating any, others accept them well. With young bunnies it’s always better to be cautious because a gut upset can be a lot of trouble.
I find that once bunnies are older and on a large variety of greens daily, that it’s much easier to introduce new greens and doesn’t require as much caution as when first starting out.