Welcome to the site.
Well you see it is hallmark characteristic feature of Bipolar disorder to have mood fluctuation between manic/ hypomanic state to depressed state and vice versa.
So the fact that he is currently in depressed state, this is expected with his background diagnosis of bipolar disorder but if he continues to remain in this depressed state for more than 1 week or if he develops severe depressive symptoms like ideas of self harm or suicide, feeling of helplessness, hopelessness and worthlessness, withdrawing himself from other people, having negative thoughts all the time then I reckon he shall have to be placed on an anti depressant drug like Lexapro ( escitalopram ) at dose of 10-20 mg / day or Zoloft ( sertraline ) at dose of 50-150 mg / day or Prozac ( fluoxetine ) at dose of 20-60 mg / day for controlling his depressive phase.
But much before taking this strategy of wait and watch and if the need arises addition of anti depressant drug, you got to get in touch with his doctor right away and ask him to add a mood stabilizer drug to his current drug regime, some good mood stabilizers are -- Depakote ( divalproex sodium) at dose of 250-500 mg twice a day or depakene ( valproate ) at dose of 300-500 mg twice a day. This is essential to control his mood swings between the two extremes besides in most cases a mood stabilizer is eventually applied as a single maintenance drug while the other drugs are withdrawn once the patient has been rendered symptom free.
So to summarize..
-- Seek addition of mood stabilizer from his doctor right away.
-- If after 1 week from the addition of mood stabilizer there is no relief or reduction in his depressive features then it shall warrant a addition of an anti depressant drug which may be withdrawn later once the depression has been controlled leaving mood stabilizer and anti psychotic drugs for maintenance purpose.
-- At all time be vigilant and monitor him and be watchful for his apparent symptoms.
I hope this helps.
Wish you all the best.