He's probably constipated with slow digestion. Everything gets backed up and food rots inside the body, creating a very toxic environment. So the body begins to excrete them out in any way it can, through the mouth and the skin. Everything about him will smell foul.
I've gotten better, but what made me sick was doctor's medicine. I tried all kinds of supplements and food cutting over the years since then, but what helped me the most was inexpensive or free. I think the vast majority of supplements don't help and they are hard on the liver. I could have bought a new car with the money I spent trying to get better.
One key is to be good and gentle to the body. He should change his diet to beat constipation, but if he doesn't, that's probably the only pill that I would recommend he take because he absolutely needs to get rid that.
I used to have problems with moist food, lettucy food, veggies, soups, and all the things that are supposed to be the healthy choices. They gave me pasty, rotten doo and the most foul Body Odor I could imagine. If that's his problem as well, have him eat more bland food--the kind doctors recommend before a surgery. The digestive system handles it better and he will smell less. Sounds contrary to everything you hear, but it is what worked for me. You can find a list by googling specific keywords.
Don't let him restrict his diet too much because he will become intolerant to the few foods he does eat and it will take years longer to get better.
Exercise. Go out and do fun things. Rescue a desperate animal at the local shelter, make new dreams, plan a new business, travel, go for walks, enjoy life.
Last, but not least: He has to remove stress from his life. If he gets upset easily, learning to deal with it is important now more than ever. Yes, stress did not cause it, but it suddenly becomes a major issue when his body is weak and toxic. If he smells and someone reacts badly, he needs to love and respect himself when nobody else will.
If he has friends or family who have toxic personalities, lose them now. He needs to be surrounded by people who truly care, even if that only leaves him with 2-4 people in his life. Would you rather have 4 quarters, or 100 pennies? Keep only the valuable people.
If he has a job where work politics stresses him out, he needs to leave. The money isn't worth it. I didn't believe that one. I was sick for 10 years and thought, "I do data entry on orders that are practically automated. This job is anything but stressful!" I kicked, screamed and got very defensive with anyone who would dare to say I was stressed. I was so convinced and stubborn. "My health is my only problem," I would say. Then I decided to go into real estate and quit my data entry job. It was only after moving on that my health attempts started to pay off. I'd removed a very manipulative person from my life who harassed me for years where I worked. I had been so used to the treatment I received, that it was normal to me. Within a day after quitting, I noticed a weight was lifted off my shoulders; 1 year of quitting, I was better. It's very simple, but so difficult to figure out because it wasn't the initial cause, just part of the answer.