Yes. I tried the separate phone but he would text both!
You need a phone that he doesn't know the number of. If you can't block, and he won't listen, you need his texts to go to a phone that is off and in a drawer somewhere. You can turn it on and look at the texts at a time that suits you. Otherwise, it's off and in the drawer.
This, unfortunately, will have to be your current number, as he already knows the number. You can put the SIM into a cheap little phone, and get a new SIM for your iPhone, and do not tell him your new number. In fact, don't even tell him you have a new phone.
You then give your iPhone number out to people you trust. It's hassle, but if you won't or can't block, then it's the only option I can think of.
Separately, you need some sort of help to deal with the anxiety - if you haven't already, see your GP and see what they can do for you. If you think of the problem in terms of things you can change, and things you can't change, altering your ex's behaviour is one of the things you can't change. He's an arse, so will deliberately text. You cannot change that. However, the feelings of horrible panic is something that you can, with time and help, change.
If, in 6 months time, you're responding to the texts with a shoulder shrug and carrying on your conversation, you can guarantee that he will be prepared to stop texting. At the moment, he's doing it because he knows it upsets you. However, that side of it, you can change. It might take a little while, but it is possible.
Really good luck with your son! I hope that the operation goes as well as can be expected. Give yourself a little break though - it's going to be a horrible anxious time, and accepting that and giving yourself as many comforts as you can will help.