If his little poop balls do actually smell right after he deposits them, he is most likely poop marking his territory, and the most likely cause for this would be because of the cat. He's letting your cat know that that's his territory.
Instead of sweeping up all his poop, maybe try leaving some in the main areas he's leaving it, to see if that stops him wanting to remark it with new poop. You can also try keeping your cat completely away from your bun and keeping your door shut so your bun doesn't ever see the cat. Then once he's able to feel secure that his territory isn't being encroached upon by the cat anymore, the poop marking might subside.
An alternative would be to try and help your bun get used to your cat, but only if this can be done completely safely, with out risk of your bun getting scratched or hurt. Keep in mind that even the simplest scratch/bite from a cat can cause a deadly bacterial infection. It's also essential to keep your bun clear of your cats litter box due to the risk of ingested litter, and toxoplasmosis from cat feces.