You don;t mention when he started taking Ritalin, but the stomach aches are a very common side effect that should dissipate after a few weeks. If they do not go away speak to your prescribing doctor about reducing the dosage or changing to something like Strattera.
If you feel he does not have ADHD, it may be worthwhile getting a second opinion, and having an in depth assessment to determine the underlying cause for his difficulties at school.
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