Hello~I would most certainly force the issue, you know your son, you know how he feels, and from what you have described, I would definitely say he needs more tests and and to be taken seriously. I would again mention the abnormal test results, and especially how your son is loosing weight and not doing well at all and seems to be getting worse right in front of you, if that doesn't help, take it to the hospital board.
I would say if they haven't already, they should check his liver functions, blood sugars, stool samples, metabolic profile, Candida, etc.
I am also thinking if you don't get results, then see a reliable Naturopath, that was my main doctor when I lived back home in the states. They will certainly run all kinds of tests that regular doctors don't even know about, will take a thorough case history and work with you. They will also test for food allergies, and all kinds of vitamin deficiencies.
I hope you get the help you are needing, it is ridiculous that the doctors are treating you like this, your sons situation, in my opinion, definitely needs more attention.