perhaps just out of shape.. but could also be early signs of asthma

Panting can also indicate a cardiac problem or overheating due to overexertion in high temps. It'd be a good idea to have your boy given a thorough physical by a competent vet.

Laurie

I have a seven month old cat who does the same thing. We've played hard (hide and seek, tag, fetch) since he was a bitty one and he's solid muscle now. Playing hard will make a cat pant as they need a way to quick process their elevated body heat. Take note of when that happens and if it's warm, don't play too hard.

I know if most people were to run at top speed while jumping and twisting around, they'd quickly become hot and short of breath. Playing is hard work. Keep doing it with him though. It's good bonding.

Laurie is right,... could be a heart condition, too, though often those cats that are symptomatic as such a young age do not do well with anesthesia, and yours survived the neuter pretty well obviously... still ,a vet visit is a good idea. I had an asthmatic cat as a kid and often would play with it until it nearly collapsed from coughing and wheezing having no clue what was wrong with it... finally the vet figured it out, but we never had to treat it... just no more chasing stuff in the grass any more.

My car has FHV (Feline Heroes Virus) a common ailment with rescue cats. Our vet explained that this is why he gets out of breath and why he snores louder than a drunken sailor!