Of those I have tried I have found that the #10 is what works. I have also had a hard time finding it without the insulation but you can buy regular electrical wire called 10-2 w/ground. You will have to take off the outer casing but there is a bare piece inside you can use. You can take the out casing off with a box cutter or a razor blade. You can just go to a hardware store and ask for 10 2 with ground and they will cut it for you.
I also wanted to suggest to you that you make one up for yourself. The Lakhovsky Coil is also great for stress and I'm sure your stress has been over the top with having to watch you son in pain. It will also give you the benefit of understanding how it is effecting your son because the coil doesn't just get rid of the pain but helps to bring your body back to normal. When he will want to get on with his life you won't be so inclined to hold him back.
Another good thing about using the coil is that as long as he uses it when he needs it he will never have to deal with this problem again during his lifetime. If the coil stops working he can just make up a new one. I have found that using the copper, after a while, it breaks down and doesn't work any more and a new one has to be made up but a new one works just as good as the first time. He will know when it isn't working as well and should go ahead and make up a new one before it totally goes out.
I hope you can get the coil made up soon and please let me know how he's doing.