Welcome to the site.
I am sorry to learn about your son's condition and I empathize with you.
You see depression per say is a condition which can be managed effectively with the help of drug therapy and counselling
, so there is a lot of hope for your son to come out of the clutches of depression and start leading a normal life all over again.
There may be different factors which could have contributed to his depression such as chemical imbalance in the brain, stressful life situations that he had endured all these years, past traumatic life experiences and so on, so it is essential to have an amalgamation of counselling with the drug therapy since he needs to learn some coping skills to withstand life stressors in the future as well.
So I propose that you get him consulted with a seasoned psychiatrist
and get him started on an anti depressant
drug such as Escitalopram at dose of 10-20 mg / day or Sertraline
at dose of 50-200 mg / day or Duloxetine
at dose of 40-60 mg / day or Venlafaxine
at dose of 75 mg / day in two divided doses, any one of these 4 options of drugs can be chosen to manage his depression with improvement in his mood and behaviour to be expected to begin withi n 3-4 weeks of starting on one of these drugs.
In addition to the anti depressant he shall be started on a specialized counselling called as CBT that is Cognitive Behavioural Therapy ,which will teach him some core coping skills and strategies to fight against stressful life situations which he is bound to face in life at some point or the other like all of us do.
So I am hopeful that with this combination of anti depressant drug therapy and counselling he shall be able to overcome his depression for good.
I hope this helps.
Wish you all the best.