Bak,
you didn't bring him to the shop to try out first? I did for see-kai-run but didn't try the pediped... He walks fine with the harder sole, but pediped is probably only suitable to wear in another 4 to 5 months time since I bought 1.5 sizes up.
I bought size 5 for see-kai-run, it is a little big for now, so when he first wore it in the shop, he kept looking at his shoes haha, then he kicked a few times and he started walking around a little draggy, then onwards, he got the hang of it and walked fine. We put him on the tricycle, and watched his leg movements, got approval from hubby to get this size and paid, haha.
What size is your boy wearing for see kai run?