ADHD is most of the time genetic, it's odd that none of your other children have it.
Most doctors lable any troubling behavior as ADHD. It's very important to have your son tested for all other physical and psychological issues before just diagnosing him with ADHD.
Check this post and follow the link to see just some of the conditions that are currently being misdiagnosed as ADHD. What is proper dosage - Concerta/Ritilan?
The boy who taught your son to do this is NOT doing this because his parents don't care how he behaves. Children do not do things like this unless they have had some sort of emotional trauma or are mentally ill.
If the school won't keep this boy away from your son then get a restraining order. This boy is obviously unstable, though it's not his fault, he may end up hurting your son in some way, and that would be tragic. If the principal won't listen go higher up.
Most children thought to have ADHD do not have it. And even those who do have ADHD can many times be treated with methods other than dangerous medications.