ADHD diagnosed in a healthy, good student & alert teen?

The teen (17) now a HS senior -had been living with us first two high school years by choice, the last year with his mother close by (the parents divorced 1990- he seemed- said he was happy adjusted - busy) -doing well (3.85 GPA) in school, social, friends, interested in many things, working part time during the summer, taking AP classes and seeming to function quite well, independent ... He is not team sport athletic but rides his bike to school, excerises at the gym and swims.  He did do a few not so good things, stole some money from his dad, and has been secretive, having his mother move here and not telling us til the day before....perhaps teen behavior - growing pains for parents, for sure!  Loved by family....Can be stubborn and want of material things, If turned down he becomes more secretive and manipulates one or the other parent to get what he wants. (my observation).  He was a bit over weight (50 lbs?? perhaps) Now in the last 2 months, and previously he has insisted he is ADHD & now diagnosed with ADHD -on Aderal - he has lost all weight -lost in the 5-6 weeks.  Can this be so simple?  & or a correct diagnosis?  He is saying he needs therapy now (psych.). His mother borderline personality disorder - frequent psych. therapy sessions.  Is there a better solution or another way to evaluate a seemingly normal healthy teen??       Thank you