He may either be too warm or stressed. Check the temperature of the water isn't too hot. Some ferrets will pant when stressed, but it can also be a sign of chest or heart problems. To be on the safe side, watch the ferret breathing when he's relaxed or asleep. Count how many breaths he takes in a minute. If he takes more than 30 – 40 breaths when at rest then a vet check is a good idea.