Before going to the doctor, wrrite down EVERYTHING even though you might not think there could possibly be a connection.
I had a steno notebook next to my desk and when I would remember symptoms, I'd write them down in the book.
Start from the head and work your way all the way down to his toes:
Foods that seem to trigger the bo as well as things that seem to alleviate it.
Gastro problems: slow digestion, rumbling noises, constipation (or alternate between constipation and diarrhea, hemorrhoids, etc.
Yes, even toe nail fungus
Does stress seem to aggravate the smell?
Does he get recurrent infections/fungus?
Has he taken lots of Antibiotics in his lifetime?
When did the odor first begin?
Does he have itching, rash, or blotches on his skin? Maybe just itching on his back?
Doctors he might want to see:
ear nose and throat
Tests they might want to perform are:
SIBO (fructose and sucrose)
Blood work, especially glucose (A1C)
Check his liver to see if he has a fatty liver
Check for tonsil stones
Check his teeth/root canals/implants, etc.
CAT scan of his nose, nasal passages, teeth, mandible
A couple of things. Appointments with specialists take months so it's best to get the ball rolling in January. The earlier the better because they'll want to see him a couple of times this year. Getting tests done early in the year means you would probably have met your deductibles/out-of-pockets early in the year and won't have to pay for anymore doctor visits, more tests, or prescriptions for the remainder of the year. Once this year is gone, then the deductible and out-of-pocket starts all over again. That can start getting expensive.