I'm with Portia. I'd ask at a local college/uni, or a technical/magnet high school with a good science program.
Higher price, unfortunately, does NOT always mean better quality, and often the 'student' microscopes with the best optics don't have lighting or apertures which are truly adequate to make use of them fully.
All of the people we know who have bought their kids microscopes successfully have done one of two things (or some combination):
a) surplus/used educational/professional model,
b) hacked/pirated components to make a Frankenscope.
Nothing turns kids off more than a difficult-to-use, inadequate microscope. Unfortunately, the 'kid/intro' microscopes are all of that ilk.
I can sometimes use those kind-- but I've got decades of experience behind me. That's the problem. Kids who lack that kind of experience actually need BETTER tools that are more forgiving. Unfortunately, you're probably talking $1500 USD for a truly
good microscope with magnification sufficient to what you're after.
Dissection scope.... well, I can recommend this--
SpiderEye Portable MicroscopeIt's a little 10X without an integrated light source-- but it is surprisingly good quality, has a nice field of view, and is EASY to operate, and-- because of the supports and design, can be used as a monocular dissection scope. DD has had one of those since she was about your son's age, and she's used it for everything from looking at
krill to examining feathers, butterfly wings, fingerprints, and crystal structures in rocks.
Get a penlight to go with it, though. More light is definitely the key to making inexpensive optics shine.
Try before you buy. I can't emphasize that enough.