Jesus fucking Christ! We're not talking about God or fairies or the Cookie monster here.
We're talking about a medical condition that is well documented, research, has a diagnosis pathway and various treatments (not cures).
It's not something you believe in or not. It's something that exists and is acknowledged regardless of your beliefs.
Maybe if you just say "I don't believe in ADHD" long enough your son will just get better OP.
So many misconceptions and myths on this thread.
ADHD is not Autism. They can be comorbid, but they're entirely separate conditions.
ADHD doesn't always mean badly behaved.
The diagnosis path is not an easy or straightforward one.
Meds can work, so can other things. Trying different things for different children is not a bad thing.
Just because a child has a shitty/neglectful upbringing doesn't mean they can't also have ADHD. Their upbringing might make the condition worse but that's it.
Just like autism, ADHD manifests in different ways in different children. Just because x does a and b and reacts positively to c and d it doesn't mean that y will display the same behaviours or respond to the same things.
And breathe...