"Testing for nephews possible condition"

My nephew has behavioural problems that are very similar to ADHD, but I couldn't understand why he was quiet at his nans house but acts out at his mothers. So I did a little research about characteristics of ADHD and found that Dabrowski's theory on Overexcitability and the Gifted set alarm bells ringing, one stood out more then most:

PSYCHOMOTOR OVEREXCITABILITY

Psychomotor OE is a heightened excitability of the neuromuscular system. This Psychomotor intensity includes a “capacity for being active and energetic” (Piechowski, 1991, p. 287), love of movement for its own sake, surplus of energy demonstrated by rapid speech, zealous enthusiasm, intense physical activity, and a need for action (Dabrowski & Piechowski, 1977; Piechowski, 1979, 1991). When feeling emotionally tense, individuals strong in Psychomotor OE may talk compulsively, act impulsively, misbehave and act out, display nervous habits, show intense drive (tending towards “workaholism”), compulsively organize, or become quite competitive. They derive great joy from their boundless physical and verbal enthusiasm and activity, but others may find them overwhelming. At home and at school, these children seem never to be still. They may talk constantly. Adults and peers want to tell them to sit down and be quiet! The Psychomotor OE child has the potential of being misdiagnosed as Attention Deficit Hyperactivity Disorder (ADHD).

My sister inlaw is looking for some support on how to get this tested as soon as possible due to the behaviour problems affecting the whole family, and not knowing how to help him through his problems in the correct way. He is 7 years old and finds joy in putting all the fish food in the tank, and then boasting about it. He is really sensitive and when someone doesn't play with him he constantly needs reassurance if they are his friend.

Please can you offer advice? Thanks.