Ignoramus9483 wrote:
My son is 8 years old and surprised me recently. He could not run all
the way up to a 30 foot tall (in my estimation) hill.

I am 38, moderately overweight, and I have not exercised much in a few
years due to so called "lack of time". And yet I could run all the way
up that hill with a 40 lb toddler in my arms, and he could not even
run up without stopping. My 8 year old son is not fat, in fact he is
skinny.

Another time he ran up 5 stories of a apartment building and again
could not run all the way, and had to breathe very heavily afterwards.

Would I be justified in being concerned, e.g. should we talk to a
doctor, or is it within some normal bounds that can be corrected with
exercise?

Being skinny does not necessarily imply being fit. I
don't see any reason to believe that there's any issue beyond
a potential lack of fitness. How active is he? How much time
does he spend in physically active pursuits each day?

Kids do go through phases, and he may have less energy
at any particular time because he's coming down with something,
or he's in that perpetually exhausted state that seems to come
with growth spurts in some kids, or whatever. It's also possible
something else could be responsible for shortness of breath, like
asthma or something, but there would be other symptoms of that.
If anything, though, he just might not be getting enough physical
activity.

Best wishes,
Ericka