hi - i've had several cats but never come across this problem, but i think you are right to be worried. i just googled this:
Panting is rapid, shallow respirations characterized by open-mouthed breathing, often accompanied by a protrusion of the tongue. It is seen commonly in dogs, but it is uncommon in cats and is not considered normal, though some cats can pant after brisk play or in times of stress, such as a car ride.
Most commonly panting is in response to environmental changes, such as anxiety, fear, excitement, or heat. However, if panting is excessive or your pet is in distress, it is important to identify the underlying cause. Some of these causes are:
Respiratory disorders
Cardiovascular disorders
Hematologic disorders
Neurologic disorders
Miscellaneous disorders
Panting may have little to no impact on the affected individual, especially in association with transient causes such as fear or stress.
If you think he is unwell, you should get him to the vaet asap or at least telephone them for advice. Don't leave him if he is obviously distressed. Good luck and update us! x