I myself have dealt with Generalised Anxiety Disorder since I can remember, but I'm no professional. I think you're a saint for being so caring, my own parents tried to help but didn't really understand or make many allowances, so already that's amazing.
So to explain what I've learnt over the years from various psychiatrists, Anxiety is kind of like depression, there's being 'anxious' and there's having 'Anxiety'. Anxiety is irrational fears, irrational worries, excessive amounts of fear and worry. Often to the point that the fear interferes with the ability to go about a normal day. DS can't sleep, so it's definitely interfering, unfortunately.
Anxiety has a lot of symptoms and causes a bunch of things. There are physical ones, like a high resting heart rate, shivering or even body tremors, sweating, the feeling of being acutely aware of your own heartbeat, nausea, vomiting, headaches. These symptoms are all caused by very high amounts of the hormones Cortisol and Adrenaline (stress and panic hormones). People with Anxiety overproduce these, and suffer the effects of that. It is manageable with either learning to control it, or medication or both - beta blockers worked for me, but I don't think they would prescribe those to a little boy. These physical symptoms are all part of a huge fear response to what might be pretty much nothing, the sufferer may not even understand why they are panicking - and so without any problem to solve it's hard to return to normal. The fear just carries on without resolution.
Talking is very good for Anxiety, it's so helpful to be listened to. As I got older, I was far better able to put into words how I felt. When I was about 7 I was able to see a play therapist, and she was absolutely lovely - it was also free. I don't know how my mum got in contact with the
kind of organisation she worked for, but I'm sure the school or an organisation like Mind could help you. Having someone be there for you and try to understand is extremely comforting to people with mental health issues. I hope I've been able to help you or answer any questions, and I wish you the best.