Brian,

"Audiologist", not kiddie-doc, LoL. Your son needs to be old enough to sit still for a few minutes for a fitting - usually about 4-5 years old. The Audiologist will measure both his ear canals, fit ear plugs if an appropriate stocked size is available (or sizes, if both are not the same size), and then stick your son in a booth and TEST according to a protocol to measure the actual attenuation. If all is well, the Audiologist will then instruct you and your son on how to properly insert the ear plugs. (And in an even more thorough procedure, they test beforehand for an absolute baseline, rather than an assumed baseline - but that's tough for a little guy to sit through twice.)

Sometimes ready-made plugs won't properly fit a youngester (or an adult, for that matter), and a custom pair must be moulded. Fine for an adult, but kids outgrow them at a pretty good clip.

BTW, that's the best procedure for an adult to go through, not just kids. Astonishing how many folks don't know how to fit themselves and don't know how to properly insert formed ear plugs. The expanding foam ones are a lot better than nothing; a lot better than poorly fitted muffs, but even THOSE have to be inserted properly to do their job. (And they're too big for little kids)

A trip to an Audiologist trained and equipped to fit and test earplugs is an investment that returns handsomely in retained high frequency hearing. We live in a noisy world...

HTH,

Tom