Whenever a doctor can not come up with a diagnosis, the patient is either suffering from anxiety or is a hypochondriac. I think there are a lot of people out there with undiagnosed ailments who are called hypochondriacs.
I was undiagnosed after slews of tests. By accident I discovered I had candidiasis (yeast/fungus infection). This causes allergic symptoms and and infestation with this pathogen is not at all uncommon. What is uncommon is diagnosing it. He can be tested for this. Candida feeds on sugars, starches, corticosteroids and antibiotics.
Get him to ask for a heavy metal bloodtest see if he has any toxins, he isnt a Hypcondriac .. they can detox him once he knows .