Welcome to YT. Sorry your little Cosmo is sick. I agree that the rosemary infusion could have made him worse. It sounds like he may be having some IBD issues.
Since your vet is stumped and doesn't want to guess, I would recommend consulting with Susan Davis - she has helped a great number of our babies. Here is her information....
Pet Health Pet Nutritionist Holistic Pet ...she is well worth the money!!!
I would also suggest getting your baby immediately on a good probiotic on a daily basis.