I agree, there does sound to be something else going on. An occupational therapist would be my first suggestion to determine what is happening with his muscle tone as this can greatly affect the ability to sit up straight for long periods of time. If a child is physically tired, concentration will obviously not be at an optimum.
Depending on the outcome of this assessment, I would suggest a full academic assessment by an educational psychologist or counsellor to determine your son's individual strengths and weaknesses.
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