Just re-read - quite long, and very big on saying 'do' and 'not do' but it is just my opinion.
I don't know if I have shared this, but I was once an in patient in a mental health hospital before they were all closed. There was a clear divide between those who wanted to get better and those that didn't. There really was, it was quite shocking.
The ex may have turned a corner with this - it is a scary realisation that you have to take a side. You can either let yourself sink or you can ask for a lifebelt. I don't think you ever were in a place where you made that choice, you were always going to try and get back on top of things even when you were at your darkest. I saw a lot of people making the choice to sink.
I have been so badly depressed that I was unable to make a cup of tea. It was too complicated for me. What helped was that I either got myself a drink or went thirsty - I was on my own. If I didn't sort out money or shopping I starved. If I messed up with my rent I was homeless. The lack of the safety net helped me make the choices to keep going. It helped me get better.
I am not saying that everyone who is depressed should be abandoned - it really doesn't work that way and if I had had some support I may not still be dealing with the fallout thirty years later. However that support is really best from professionals.
He has turned to you when he was in a desperate place. The trouble is, in the past he is used to you looking after him and he will slip back into that groove in his life where he relies on you. He needs to work out what to do without you being his safety net - with appropriate medical support. It's great that he is seeing his GP but if the meds aren't working after 5 weeks then they need reviewing and he needs to tell the GP this. He needs to tell them how bad it is and get support in place, get referrals, get other fallbacks - and they are easier to get if next time he does this (he probably will) you call the police.
This is not going to be an easy time for you, and if DS sees too much it can do permanent damage. Wishing you all the luck in the world.