It could be stress, but it’s a bit abnormal for a bunny not to poop in 12 hours. Are you giving him hay? Hay should be the biggest part of his diet and he should always have it available unlimitedly. If your’e not feeding him hay, then that’s why he’s not pooping. For hay types, when my bun was 8 weeks old, I fed him timothy hay and some alfalfa hay to mix it up a bit. They say baby buns should have alfalfa hay as their main hay, but I personally believe that grass hay is better. You could order hay online, or get it at a pet store. Even Tractor supply has hay for small animals. Since he’s still a very young rabbit, feed him pellets. Oxbow sells pellets made for young rabbits. I fed my 8 week old rabbit a 1/4 a day of his young rabbit pellets. Too many pellets isn’t good. And for veggies, you can start feeding them at 12 weeks old which is at 3 months. According to the House Rabbit Society (they study and know rabbits from head to toe) rabbits should have fresh greens when they’re 12 weeks old. Just make sure you introduce very slowly. Romaine lettuce is good to start with. You have to introduce only one veggie per week. So for example, give a small piece of romaine the first day, and increase the amount a little bit each day. There are more forums that give more detail on introducing veggies. Again, make sure he’s eating unlimited amounts of hay. Good luck