I think you need to consider the possibility that he doesn't appear to have problems at home because you are exceptionally good at managing and preventing issues. Grinity, who doesn't post much anymore, mentioned more than once that you should never underestimate the ability of a gifted parent (mother) to compensate for their 2e child and make them look "normal" at least some of the time.
I know too many kids wih ASD or ADHD who either behave impecibly in the world and then fall apart at home, or are ok at home but not at school to have faith in the "multiple environments" thing. Sure if he's fine with all teachers except one then its probably the teacher.
Also it's common for kids with ADHD to have either (or both) WMI and PSI low, but neither are a requirement. My DD has ADHD and has WMI in the HG range! And yes she absolutely has ADHD and you can see that the WMI is high in every day life. Possibly her processing speed is low but it's never been tested accurately.
Also ADHD is far more heritable than asthma or breast cancer. If the tester WAS biased its not without reason there.
By all means get another opinion, but please consider the possibility this is real.
