Ideally a rabbit should eat hay, fresh twice a day as big as his body, a teaspoon of high fibre pellets morning and evening, and just a handful of veg DAILY. If you don't feed veg daily, they never get the gut bacteria to digest it, so it will always upset their tum, especially if they don't eat hay and have loads of mix to eat, as there won't be a lot of fibre in their tum.
If your rabbit's eating the right amount of hay and pellets, and you want to give veg, including grass, then just give a mouthful at first - tiny piece, and slowly increase this over the course of 2 weeks, assuming it doesn't upset their tum. This will give the digestive bacteria a chance to build up to sufficient numbers to cope with digesting it