Congratulations on your new bun! He is adorable!
To keep him healthy, slowly and gradually switch his food from the seed and corn feed to healthy rabbit pellets. There are several good brands that make junior pellets suitable for young buns. Many of our members use Oxbow, if you are in Europe there are other brands that may be easier to get.
As for hay, grass hay is the most important part of a bun’s diet. All grass hay is good (meadow is a mix of many types of grasses), but baby buns also benefit from alfalfa hay. If a young bun eats lots of hay it helps making their gut microbial flora robust and diverse -and that’s the best GI stasis-prevention there is. Hay wears the teeth (rabbit teeth grow all the bun’s life and need constant wearing down), that’s very important for tummy health as well.
The HRS suggests introduction of greens at around week 12, unless the bun has had greens while still with its mother and siblings and seem to be doing good on that diet.
Here’s a link to this site’s info on a healthy bunny diet: https://binkybunny.com/BUNNYINFO/tabid/53/CategoryID/2/PID/940/Default.aspx