“What is the best treatment for bipolar depression?”

Medication. There are great new medications and are very effective for Bioplar Depression, and therapy would be appropriate.
Hope that helps.

There are quite a few treatment options available both in terms of therapy and medications and your son's psychiatrist would be the best to decide as he would know complete the history of your son's illness.
However, as per our current understanding through the medical literature, Latuda and Lithium are the two top choices for bipolar depression among few others.
hope it is helpful to you.
Regards

Right now there are no head to head studies looking at best treatment for bipolar depression. However, there are several medications that are indicated specifically for bipolar depression. The best approach is to work with your psychiatrist on a medication that would work for your son with the least amount of side effects if at all.