Hello Asterix,
I will pray for your son. I also recommend that he see a physician, if he has not done so already. Depression is not "all in our heads," as some people claim, but it can be a dangerous disorder that keeps a tight grip on someone until the Lord heals or until it is managed. There is some excellent medicinal therapy available, with few side affects, that might help your son until God heals him. I am not a medical professional, but I have dealt with depression for many years within my family and have seen some healed and some manage their depression with medication.
My prayers are with you and your son.
Blessings,
Lynn