The mothball-like odour is caused by mouth bacteria and it is difficult to eliminate it by rinsing with over-the-counter mouthwashes. This strong odour often is found between the teeth but even with flossing it tends to come back quickly. He should gently scrape his tongue and then brush it with the mouthwash. If this is not successful, go to a dentist or clinic. Summarized from this source: http://www.breathdoc.com/faqs.html#04

I'm posting this in the hope that it might help keep someone else from going through about 2-years of misery like I did... I developed this problem myself and despite a clean bill of health from the dentist, thought it was a tooth problem or food stuck somewhere because that is the usual cause. I would brush, floss, and even purchased a WaterPik (which I highly recommend) to try and get rid of the problem, but it still persisted... in fact, I walked out of the dentist's office one day after a thorough cleaning with that mothball taste still in my mouth. I also developed this maddening syndrome of low-grade fevers and flu-like symptoms that would continually go away and come back... sometimes they were bad enough to seriously interfere with my work. I went to several different doctors and even to the Johns Hopkins Division of Infectious Diseases to no avail... no one could figure out what was going on. It really FELT like an infection and in desperation, after about a year of having this problem, I asked my usual doctor about a trial of antibiotics but she wasn't willing to prescribe any and sent me out for more scans and tests to try and figure out what was going on. After all of these tests turned out to be negative and I ran into the dead end of "we don't know what's wrong with you, there isn't much we can do"... it was at this point, with visions of some rare tropical or lyme-like disease such as Brucella, I decided to try some antibiotics on my own (you really shouldn't do this - don't try this at home). The antibiotics helped a LOT but the symptoms eventually returned after a few weeks albeit at a much lower level. Still not knowing the true cause and with misguided thoughts of some rare disease, I actually considered taking a 6-week course of Doxycycline to try and wipe it out but that's a drug with potentially serious side effects and allergic reactions that you really don't want to mess with. Since the symptoms were very mild at this point good sense prevailed, and I decided to just ignore it and hope it would just go away or that the solution would eventually present itself. I started suspecting that I might have a sinus infection and finally, one day when my poor wife told me I was having that problem with my breath again (the mothball smell/taste generally got worse if my nose was running or if I reclined/laid down), I asked her if the smell was also coming from my nose if I breathed through it, and it was. At this point I called the doctor and while I wasn't showing the normal signs of a sinus infection, he thought it might be deep within the sinuses and prescribed a strong dose of Augmentin XR which finally eliminated the problem. So... after dealing with this problem for about 2 years of my life, it turned out to be a sinus infection! At least it's over. I should also mention that the usual blood tests for infection/inflammation such ESR, CRP, etc. and even an Indium white cell scan were all negative as well, probably due to the fact that the symptoms would come and go and I usually felt better when I went in for the tests so if this happens to you, don't give up. If one doctor won't take you seriously, then try another. - Phil