Hay is critical for a rabbit to stay healthy. I don’t find mine cares less for fresh grass. Yes, cabbage can cause bloating, as can other veggies, such as broccoli; however, small portions are fine. You have to see how your rabbit reacts. If it affects him badly, don’t feed it to him. Mine loves broccoli and has no ill affects. He also has no problems with cabbage-like foods; e.g., radicchio, bok choy. Veggies with high water content, like lettuce, can cause diarrhoea. Introduce new veggies slowly and just one at a time so you can see what happens. Re lettuce, only feed dark green lettuce, it has more nutrients. The best is romaine. Head/iceberg lettuce should never be fed, as it has no nutritional value.
If your rabbit does react (gas, diarrhoea), stop that veggie ’til he gets back to normal and later try a smaller amount. It always worries me when people believe they shouldn’t feed their rabbit any veggies because of diarrhoea. Veggies are critical to their health. You’ll just have to see which ones work best for your bunny, and introduce them slowly. Also, some veggies are toxic, so always check first (there are lots of sites on the internet listing safe and toxic foods for rabbits – just google it).
Hope that helps.