You can test that very easily. Get a cheap pair of paper-framed
glasses (they usually cost a couple units of currency, wherever you
are) and ask your son to look at them and tell you whether he
perceives the colors of the two lenses as distinct — which will only
be important for viewing color anaglyphs — and more importantly,
whether they seem to be of comparable brightness. If one of them looks
much darker than the other (to the point he can hardly see anything
through it), then it will not work.

Even an image such as this can be a good test:

http://www.underground3dmovies.com/anaglyph%20glasses.JPG

The typical lcd dyes in your computer screen will have similar
spectral windows to those in the anaglyph glasses, so if he sees them
as differently and neither is too dark, go ahead and buy the glasses.
You will find plenty of anaglyph images on the web to test against,
once you’ve got the glasses.