Dear Marc M.,
The diagnosis of Bipolar D/O, or any Mood D/O for that matter, in such a young child is a somewhat controversial subject. However, it is clear that some very young children do display such conditions, and if the diagnosis is correct the need for medications (antidepressants, mood stabilizers, anti-psychotic agents) will likely persist because Mood D/O's are biologically-based conditions.
Because the stakes are high, and the ramifications of taking mood stabilizing drugs and anti-psychotic drugs are significant, it would make sense to arrange a second opinion.
If there is a family history of Mood D/O, and particularly Bipolar D/O, your son's vulnerability certainly increases. At his age, while therapy for him can be useful, you will achieve the most mileage from the medications and from a systematic plan for behavior management within the home.