L.M.
I think kids are just like grown ups. They are all in different levels of being in shape. I also live in a cold weather climate, and I realize that in the beginning of summer, the kids are more breathless and after we've been physically active for a while they build up their endurance for physical activity, just like grown ups.
Don't ignore it if you feel like there is a physical issue - like he wasn't breathless and now he is, for instance, but also realize it's a couple months into a cold weather season and not as much running around is being done.
If the doctor says it's ok, maybe enroll him in some indoor sports for the winter season.