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.
Turnip greens are technically cruciferous (which means they can cause gas) and rabbits sometimes have trouble handling gas. Some rabbits are far more sensitive than others. That said, I've fed tons of turnip greens over the years without issue. Just be aware of signs of potential gas when introducing them for the first time - hunched over, signs of pain, reduced appetite, reduced volume of fecal poops, etc. If your rabbit seems 'off' after feeding turnip greens, treat the issue with simethicone (in the form of baby gas drops from any pharmacy or grocery store) and reduce or eliminate that veggie in the future. As long as you don't see any warning signs of GI stasis/tummy troubles, turnip greens are safe to feed to your rabbit (I've routinely deposited up to one (store-bought) bundle per rabbit at a time into one of their enclosures without issue).