laska, I got the embroidered jumper which is ££££ and will only stay if it's utterly perfect. The diamond sweat top and cropped flare jeans. I'm not counting any of these as spends as I simply don't know if it will all go back, it often does..
sunny. I'm not sure what to suggest really. A runners shop would do a gait analysis on an adult, I don't know if they would on a child. A physio would as a matter of course if the person displayed an injury, to identify what area of their body wasn't working quite correctly to cause the problem. Again, not sure if they would as for a child. I could suggest also though an athletics club with a specific junior section. They are pretty low key and focus on enjoyment over technique at age 8 but an athletics coach could give technique advice. How's your DS's fitness overall? Is he sporty and outdoorsy in general? My DS kept up because he was so fit in general. And that might be something you could help with?
That being said, DD was never the fastest and never so interested. We made sure she had a good level of general fitness by doing lots of outdoor stuff and a few clubs and activities and let her find her niche, she often chose to hang around with the quieter kids and do other types of stuff.
I guess it depends on whether your DS really wants to be speedy and join in, or whether it's better to encourage him to do other stuff. Which only you can judge particular to your DS. Some kids (like DD) were never made to be the fastest and that was just genetics sadly. 