Is this something that seems to amuse him, or that seems to be part of an examination? For the later, does he then study the glasses or the face? If he seems fascinated by the glasses themselves, he could be trying to figure out how they work. If he then studies her face, he could be trying to understand what her face looks like without them, which could be curiosity or related to some version of face blindness.
I'm going to guess it's pretty simple curiosity, however. I have a vague memory of my NT daughter going through a short phase like this around that age, maybe ....
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).