Cute little guy
Yeah, cat litter isnn't good for them. Clumping litter is really bad, so if it's that you'll want to stop using it immediately, as it can be deadly to rabbits. Cat litter can also cause respiratory problems. His sneezing problem could have something to do with the litter. Try one of the ones suggested and see if the sneezing goes away.
If the probiotic that your using has dairy in it, that could be causing the weird poops. Don't give it to him and see if his poops are better tomorrow.
I'm not sure where you should go from here. With him not eating and drinking like normal, that means that something is still wrong with him. If it's a bladder sludge problem then hold off on the kale for a while and give him green leaf lettuce and cilantro instead. Kale is high in calcium and can contribute to bladder sludge problems. You could try that and see if it helps. It's also possible that parasites could be causing the problem, but you'd need to get a fecal done at the vets to know for sure. If you do take him back to the vet, definitely try to find a better vet to go to.
If the probiotic that your using has dairy in it, that could be causing the weird poops. Don't give it to him and see if his poops are better tomorrow.
I'm not sure where you should go from here. With him not eating and drinking like normal, that means that something is still wrong with him. If it's a bladder sludge problem then hold off on the kale for a while and give him green leaf lettuce and cilantro instead. Kale is high in calcium and can contribute to bladder sludge problems. You could try that and see if it helps. It's also possible that parasites could be causing the problem, but you'd need to get a fecal done at the vets to know for sure. If you do take him back to the vet, definitely try to find a better vet to go to.