Hi, these are great questions! 
My degree is in special education and my focus is ADHD and similar disorders. I’ve asked this question of several of my professors and the answer is pretty consistent: there will be some initial physical growth inhibition but the child will catch up by adulthood. The impact will be lessened by the same things it is for normal children: limit caffeine intake, make sure the child gets at least 8 hours of sleep per night, proper nutrition is key, make sure the child drinks enough water, etc.
From what I remember when I was younger, my brother was on this medication and there wasn’t really a noticeable difference. He was still within the normal spectrum. He might have ended up a couple of inches (that little of a difference!) taller if he hadn’t been on the medication.
My recommendation is don’t take him off the medication. The growth inhibition is not a noticeable amount, but the behavioural difference is definitely something that gets noticed! Also, when he is an adult, which is going to make a bigger difference: that he’s 5’7″ or 5’10” or that he is someone who believes in himself, has a good support network of friends, and experiences academic/career success?
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