As Jane said, the baby needs a vet ASAP.
When i worked at the pet shop we had a baby bun with severe diarrhoea. It took 24/7 nursing (and i mean 24/7) to get her through it. The vet gave her fluids and meds and i was feeding round the clock, giving meds and tummy rubs. I really hope the wee bunny will be ok, good luck at the vet.