How do you know if your child has ADHD?
Hi, and welcome!
You don't mention his age, but I am guessing elementary age.
If you suspect ADHD you will need to have him evaluated by a Neuropsychologist. They will also test for other things, to rule out stuff that can mimic ADHD. It is possible it could be omething entirely different.
A child may HAVE ADHD, but it is not what they ARE. Never tell a child they ARE ADHD.