From what I gather, Nortriptyline can't be any better than something like Bupropion (Wellbutrin) or Effexor for treating ADD. The SNRI effects of this TCA help ADHD, but the major downside is that it is a mACH antagonist. Acetylcholine agonists help with focus, not antagonists (anti-cholingergenics).
Also, MPH is a relatively powerful stimulant, not as powerful as amphetamine, but still quite powerful compared to SNRI's, NRI's, or TCA's. I still think his doctor is being investigated, or suspects abuse, and wants to prescribe a non-scheduled drug instead. I see something like Wellbutrin being far more effective, as it reuptakes and releases DA at a 2:1 ratio relative to NE. Also, it lacks mACH antagonism, which would mean no sedation, foggyness, memory deficit, focusing problems, etc.
Let us know how it goes, OP, but I would try to switch back to Ritalin immediately before you become dependent on an anti-depressant. I think these are more for hyperactive children who need some sort of significant serotonin activity and mACH antagonism to relax them. It should be used for a non-anxious, non-depressed adult who has had good results with Ritalin for 14 years, imo.