He'll be fine without pellets, but you need to feed him something or he could very well end up sick and develop gut stasis and a blockage. It's not good to even let a rabbit go more than 12 hours without eating, let alone more than 24 hours without food.
If you have long course fresh grass, that can substitute. Early growth fresh grass may be ok, but there is more of a chance of it upsetting his digestion if he isn't used to it. You will have to cut a lot of the grass If he is used to veggies or forage, then you can also feed those.
He needs to have food, and enough of it to keep his digestion moving normally.