Please tell me they tested him for celiac disease which IS a malabsorption disease. I can not believe they would tell you that and not tell you how to deal with it.
I have multiple family members with this disease and there is no cure or medicines to fix it. A life long adhereance to a gluten free diet is what will cure him. He will start to put back on teh weight, the stomach pains will go away and he'll feel like a whole new person. This may not happen right away but it will happen. Gluten is the natural protein found in wheat, rye and barley & must be avoided. The foods you describe, while wonderful, attack the intestines of someone with celiac disease and this may be why you aren't seeing an improvement in him.
It is something he was born with & it's hereditary. But it seems that sometimes in people with celiac disease, they are going along in life pretty much ok then something happens & the celiac disease is triggered & they start getting very sick. Weight loss, diarrhea, loss of hair, stomach pains, and gas are just some of the symptoms.
If they have not tested him for it, INSIST on it. Blood test are generally the 1st step and aren't always accurate. Endoscopies are the preferred method as they can check for blunted villi. Villi are the small fingerlike things in your small intestines that take the food and dispense it to the body. In those with celiac disease the body treates gluten as an invader and it causes the small intestines to lose that villi and become smooth. Therefore, food isn't being absorbed at all. Once on the diet, the intestines begin to heal but don't be fooled into thinking he's over it. It will always be with him. Also, does he have an issue with dairy? Most celiacs are lactose intolerant and over time that improves too.
I am the President of the Memphis Celiac Support Group and would gladly give you more information if you so desire. He needs to know this is NOT the end of the world. We found out my 19yo daughter had this when she was almost 18. She has never felt better in her life and wouldn't go back to eating gluten in all the world.
You may email me privately at ____@____.com and I can give you information about our next meeting, scheduled for Monday the 19th 7PM at Wild Oats on Poplar Ave.
In the meantime, here are some websites that may help you:
http://csaceliacs.org/
http://www.celiaccenter.org/default.asp
http://celiac.org/
Sorry if this was too long but your post sent red flags flying all over when I read it. If your doctor doesn't know about celiac disease find one that does. The sooner you know the quicker you can adapt his diet & get him on the road to recovery.