Thanks for that additional detail. Please understand that online I cant state anything with 100% certainty, I can only give my best educated guess based on what you wrote. I am so sorry for your father and I hope I can help give some guidance. Despite what you wrote about nothing changing even though he cut out wheat, I am still wonder whether he may have something called celiac disease, which is an allergy to gluten.
Gluten is found in more than just wheat; it is also present in barley, rye, and possibly oats. The symptoms of celiac disease are very much like what your father has been experiencing, the bloating, the gas, the weight loss. People with celiac disease can also have lactose intolerance, but not all will.
Other symptoms of celiac disease include fatigue, easy bruising, itchy skin, cramps, and allot more, and is due to the fact that the gluten allergy and its reaction in the intestines prevents the body from absorbing needed nutrients. People with celiac disease can have constipation or diarrhea, and sometimes have stools that float and smell truly foul.
There is a blood test for celiac, its a transglutamine antibody test. You may wish to speak to your doctor regarding that.
More information on celiac disease is here: http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001280/
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I hope I was able to help, and that your father feels better soon.