8 year old boy cannot run up a 30 ft high hill

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?

i