What blood tests have been done?
TLI with vitamin B12 and folate? Spec cPL? Triglycerides?
Have PLE and Lymphangiectasia been ruled out?
Sadly sometimes steroids are the only thing that works and if those are no longer able to be used, I'm really not sure what can be used.
Have you tried probiotics and maybe digestive enzymes?
How about a homemade hypoallergic fish and potato diet?