ADHD medication targets the dopamine brain neurotransmitters where antidepressants target the serotonin and norepenephrine. That is how they differ. If he is not responding to the medication for his ADHD symptoms, what you're observing ex: inattentiveness, hyperactivity/hypomania may in fact be related to a mood disorder such as bipolar. At least, you would want the latter to be either diagnosed properly or ruled out. Consulting with another psychiatrist specializing in mood disorders besides ADHD may be helpful.