It looks as though your son may have mild ptosis or drooping of his left eyelid. This can make the eye appear smaller, though it really is not. Ptosis can often be surgically corrected.
Please google or Google-image the word ptosis for photos and information.
Second, it's not clear in the photo, but has he ever been told that he has a slight strabismus, in which the eyes do not point perfectly in the same direction? It could just be something about the camera angle, though.
Third, these issues--and your son's corrective lenses--are things that he and you should see an ophthalmologist (not an optometrist) to talk about.
I beg to differ but a good optometrist can diagnosis all these conditions. Optometrists are the experts on binocular vision testing and refractive problems.