Moderation, moderation, moderation! Introducing a bunny to new greens should be done in very small doses, especially because turnip greens are so controversial to begin with. As long as you watch how your bunny reacts to them and feed carefully, they should be perfectly fine. But do take care in where you get them, make sure they are pesticide free! I like to visit my local farmer's market to get greens for my bunnies, but they're spoiled rotten, haha.
Your bun may be a bit too young for large doses of greens. I would stick to maybe a few small pieces of romaine lettuce first, but as the bunny gets older, you can introduce more mixed greens such as turnip and other collards.