We've got an appointment with the paediatrician in November, so I will mention it to her. It is an obvious difference between him and his peers. He is the first one to get tired and need a rest. It's the same if we go for a walk.
My husband tires easily from walking. I think he has low muscle tone probably since he was a kid (I've known him since we were 14). Also another thing to consider is that men typically have more upper body strength than women do and women tend to have more lower body strength than men. So women can go walk around a mall for 4 hours shopping and their husbands tend to give out from achy feet around 1 hour. Or less if they really hate shopping. Now, the lower body strength thing is not true for all men, just is more common... maybe have something to do with their center of gravity and their muscle tone and that women do tend to have thighs and hips and all that. I don't know... just speculating. I guess if a man/boy has low muscle tone, maybe he'd tire more from walking?? Really, I'm just speculating... just try to get him working on his muscle tone by getting out and exercising. You have to start small and work up. If you force him to a level he's just not at... he'll probably resent exercise at all. That's where I'm at with myself... not the low muscle tone... just being out of shape. LOL...
Maybe my mom remembers more about this than I do, but for me I noticed a change around puberty. Even as an adult I tend to tire pretty easily.
I remember as a child not doing very well in phy-ed. It was one of my least favorite classes because it took a lot of energy to run. Even though I seemed to have less energy and require napping after puberty I did become better at running (sprinting). That's just an interesting note.