I started reading this thread in complete horror at the ignorance and the attitudes displayed, and then read about your poor ds and how scared you are.
I am so sorry to hear about this, it must be an absolute nightmare for any parent. I was severely anorexic in my twenties. I'm nearly 40 now and a normal weight, I still have some issues about food but no one would ever know I had nearly died.
The first thing you will need to understand is that it is not about food. It's really not about food. that's just the symptoms, really, it's how it is expressed but it isn't behind it, and it won't cure it just to make him eat. You can't anyway of course.
My biggest bit of advice is NOT to ask, encourage or tell him to eat, at all. In fact don't even mention food. Every time you mention food or eating, that will make something switch off inside him and he will refuse to eat even more. That's because it's someone trying to take his sense of control away - however well meaning they might be. He HAS to stay in control or he won't do it.
Concentrate as much as you can on his feelings and what is going on inside of him, what is making him scared (as well as the food - there will be something else, he might not realise what, though) and keep him talking. But just avoid the subject of food at all costs.
Let him know you love him and think he is lovely, and beautiful and great fun and that you know he is not happy, and you will do whatever he needs you to, to help.
Please keep posting if you find it helps - we are here and waiting to talk you through it. There's quite a lot of us on MN who have come through this.