I give my rabbit two cups of greens twice a day. If you have never given veggies to your rabbit start off with one vegetable at a time. They have really sensitive stomachs and some greens might give them diarrhea. After 2-3 days of giving him the same veggie and if if he is pooping and eating normally you can try feeding him another veggie. NO Iceberg lettuce that gets them really sick!
Try Romaine,Red Leaf,Butter and Green Leaf lettuce
You can also add- cilantro,parsley,dandelion greens, arugula, fresh mint.
This should be given as treats. A small piece of carrot since they are high in sugar, small slice of apple(no seeds= can poison them), small piece of banana, piece of pineapple or papaya.