First, I don't know how old your child is ...as that makes a difference.
Yes, he could have normal results since this is a test designed to be "used for assessing the core symptoms of ADHD: hyperactivity, inattention and impulsivity." Furthermore, the makers of the test say that, "The QbTest results should always be complemented by a clinical interview." Did these doctors interview your child or give any questionnaires to you or his teachers?
I think it could very easily not catch a child with ADD. The gold standard for diagnosis is to do a questionnaire with the teachers and parents. ADD and ADHD are very different. Good link here - https://helpguide.org/articles/add-adhd/attention-deficit-disorder-adhd-in-children.htm
My personal opinion is that this test is just a way for inexperienced professionals to deal with a problem they have little knowledge of.
Are you in the United States (which state) or England?
These doctors are not going to be helpful. You might have more luck going through your local schools. Your best bet is to find a doctor that specializes in ADHD - probably a psychiatrist.
The fact that you are asking these questions and willing to go online for answers, shows me that you have a very legitimate concern. If I can give you any more information, please ask. But, I would certainly give your concerns to his teacher. Best wishes.