Hi there
Some rabbits are more sensitive to greens than others, and as such don’t tolerate greens being introduced to their diet before a certain age. Much of this is likely depending on what the mother rabbit ate, and whether the kits got to nibble any of it while growing up.
At 12 weeks, you can certainly start to introduce greens. I would pick one thing only and offer a very small piece (size of a pinky finger), then observe for 24 hours to check for gut upset, then offer a slightly larger piece if all is well, and so on. If the greens don’t agree with him, you will likely see some mushy poops, in which case, stop the introduction, wait a week and try again. My previous bun Bandit I got at around 13 weeks and he couldn’t tolerate greens, I think after a break we managed to introduce them at about 5 months of age. After a week or so of introducing the first green and it going well, you can pick something else to introduce as well, starting small again.