I’m very sorry for your loss of your bunny. 🙁 He was very young, so there are a lot of things that could have gone wrong. It’s possible he wasn’t weaned correctly or was taken from his mother too young. Large shifts in diet can also cause issues. He also could have had coccidea, a parasite which causes diarrhea and is often fatal in baby bunnies. In all rabbits, diarrhea is an emergency, and in young rabbits especially it can be fatal very quickly.

For future reference, young rabbits (under around 6-7 months of age) should have a diet of unlimited alfalfa based pellets and unlimited grass hay. Timothy pellets are OK, but then they need alfalfa hay as they do need at least one source of alfalfa, and pellets should not be rationed tightly when they are that young. Rabbits need hay especially available at all times, as going more than 12 hours without eating can cause serious health issues. Around 6-7 months old most people start reducing the amount of pellets. Depending on the breed, alfalfa may be needed for up to a year (for large breeds), but most are done growing around 7 months so you can switch to only timothy hay and pellets.

If you choose to get another bunny, I would encourage you not to support that pet store as they didn’t give you good information and may have given you a sick bunny. I always encourage people to look for rescues or shelters in their area as there are so many wonderful rabbits that need homes.

I’m very sorry this happened. 🙁

Although not the recommended type of pellet, the timothy pellets are probably what he was eating before you got him. So I’m going to guess that there was something else wrong. As Dana said, young bunnies are very susceptible to parasites like coccidia. It takes awhile to incubate, so it would make sense that you didn’t see symptoms right away.

Dana gave good advice regarding proper diet (for future reference). And yes, be very careful about where you get a bunny. It isn’t unusual to find pet stores selling sick animals.