D.B.
It could be that his eyes are getting use to working together still since getting his glasses, but what Ive read and my first thought (my background being in the eye field) was to go have him check out at an Pediatric Ophthalmologist.
They will be more detailed, and specialize more than a Optometrist.
If you cant find a pediatric Ophthalmologist, dont be afraid to ask other Ophthalmologists how many children they see a week or a month. You want someone whom will be good with children, patient and will take the time to explain everything to you also. Ask around and find one that has a reputation of not being one of those doctors whom are always in a hurry or gives you short answers.