You’re right- instant release pills usually are given twice daily, but some people find that they only need the medication for part of the day, or that taking an additional dose later interferes with their sleep, etc. etc.
If you want him to have the same effect throughout the day, he’d need to take an additional pill at the same dosage, or a smaller dose in the afternoon, which wouldn’t give the SAME effect, but it would give more effect than unmedicated afternoons, if you understand me.
To be honest, higher dosages are only harmful if it’s more than he actually NEEDS. He’s not going to turn into a junkie or anything just because he’s on a higher dose, AS LONG AS it is the dose he requires, and not just extra medicine for the sake of it.
The reason pills are normally given multiple times a day is to maintain whats called ‘Bioavailability’. If you take one 10mg pill in the morning, and one in the afternoon, your body always has 10mg available throughout the day, even though the daily dose is 20mg. One 10mg capsule would, by this logic, have 5mg bioavailability throughout the day. The fact that your son responds well to 10mg of bioavailability suggests that it’s a good dose for him, and you can give him that dose AGAIN later in the day. It won’t increase his tolerance to the medication, because you’re giving the second one when the first one runs out. Make sense?