On MummaB I am soooooooo sorry you and DS are having such a rough time...
I cant help with the next step after losec, but wanted to offer my support. My Paed didn't go any further into it than that with me.
Have you tired using the formula thickener, but giving it to your DS while breastfeeding? My Paed did suggest that that might be an option, but cautioned me that it can case some constipation, so advised me to make it up half strength and see how that worked. I had a look a the tin in the chemist and it does have directions on it for use with breastfeeding, but altering my diet has helped my DS enough so at this stage I have not tired this, but it sounds like you need to "pull out the big guns" now.
Also, with my DS I have cut out all "acidic" foods such as tomato, orange juice, and other reflux foods like chili, and curry and anything spicy, and I don't have any cafiene.... For me, this helps with my DS. I think other things you can eliminate from your diet are gluten, and wheat, as they can have an impact on reflux too.
Hang in there hun. I am sure, somehow, you will get through this.