Why won't my rabbit eat his pellets anymore?

Several things could explain that. First, check if the pellets aren't too old or stored badly - they lose their smell and taste pretty quick, especially if they've been left open to air or in direct sunlight. Then, maybe your rabbit has dental problems (teeth growing too much, that's common in rabbits) or an infection, so a vet check would be a good idea, especially if you notice other behavior changes. It's also possible he just discovered something more interesting to eat, like hay or fresh veggies, so try mixing things up and see how he reacts.