A lot depends on what your son's rx is and whether it is changing slowly or quickly.
I had a local optician, who specialized in custom mask lenses, make up my rx and bond them into my existing mask, years ago. He needed the interpupilary distance (your ophthalmologist can tell you this but it should also be written on your son's rx) to make sure the lenses are centered properly. An off-the-rack mask doesn't accommodate for this at all. And he compensated for the fact that the lenses would be "out there" against the glass, not fitted up on the bridge of my nose. So, there's some skill involved that "off-the-rack" won't get you. If you do this, I'd strongly suggest getting a set of blank lenses for the mask, to make SURE you can replace them when his rx changes.
I love the mask but over the years that rx became useless, and no one could UNBOND the prescription lenses, and replacements were no longer available for the glass in the mask. Except...those wonderful prescription mask folks in Cali! They had no problem actually cutting and tempering two new lenses for the mask, and I now dive either with contacts or with fuzzy distance vision. GREAT FOLKS and timely service, another voice from the choir!
Now, if your son's vision can be expected to change every year or two and you don't want to keep replacing the mask as he grows...by all means try off-the-rack and whatever comes close to his rx. The vision won't be perfect, but for diving, you can get away with that. Like reading glasses from the grocery store...it still beats all heck out of nothing, and can really impact the cost.