As someone who had a breakdown when I was fifty, I can speak with some experience.
Having an understanding employer is a great help.Your Son has done and is doing all the right things. Admitting to himself that he needs help is the first step to recovery. There are excellent support services via your doctor and NHS. I did find that there was a long wait for NHS counselling in my area and went to a private councillor at first. Councillors seem a waste of time at first, just chatting and asking unrelated questions, but it helped me no end.
Be prepared for a long haul, took me 6 months to get back to work.
As far as your reactions, you seem to be getting it right, gentle support try to accept the low moods as part of the depression that just have to pass of their own accord. Try to talk to others that have suffered from depression to get to understand how he feels.
Medication, your Dr, may recommend medication, lots of for and againsts discussed online, trust your Dr.
I have been on medication for 20 years, keeps me on a level keel most of the time and does not have any significant side effects.
From your post, I'm sure you will all get through this soon.
If I can help, PM me.