It may be from stress, but it can still be potentially fatal. Is he still grinding his teeth? Have you been feeding the same kind of food that he had at the breeders? Changing food can sometimes cause problems. Do you feed him hay? Sometimes they will still eat hay even when they won't eat their pellets. Normally you don't feed babies veggies but you need to try and entice him to eat. You could try a small piece of green leaf lettuce or a sprig of cilantro. You could also try a just a few rolled oats. If you can't get him to eat and he has stopped pooping for several hours, you need to get him to the vet immediately. Rabbits develop GI stasis which can kill them. If he is grinding his teeth, he is in pain from something. It could be from developing gas, which will cause a rabbit to stop eating. You could try giving simethicone (infant gas relief liquid). The adult dose is 1cc every hour for 3 hours and every 3-8 hours as needed after. So maybe 0.2cc's for your baby. You could also try syringe feeding him some food. You can make a pellet mash by soaking his pellets and mashing them up. You need a syringe with a large opening for this. You can also get plain canned pumpkin (not the pie filling kind), and syringe feed that. He really needs food to keep his gut moving. It's not good for them to stop eating for long. He also needs to stay hydrated. If he's not drinking then you need to syringe him water too. Do it very slowly into the side of his mouth so that he doesn't accidentally aspirate any of the liquid.