When you say almost invisible, can you describe it in more detail? Is in inside or outside the mouth or both and what are the symptoms?
He should definitely go to a doctor as without a diagnosis he is clutching at straws as to a treatment and could make it worse.
Doctors can be very good at spotting signals so they may be able to help guide him in the right direction as to the drinking and smoking or at least give him the know-how that help is out there is he wants it.
It could be something like gingivitis, especially with him being a smoker and picking things up off the street and possibly coupled with poor dental hygiene.
One idea would be to use a good antibacterial mouthwash in addition to the usual toothpaste along with flossing to remove anything nasty from in there.
He should take care though if he has sore or delicate bits in his mouth and a trip to a dental hygienist for a proper clean and check up may be a good idea.
Oral thrush is also possible as is a general infection.
Get him to see his GP and, if he is looking for any help in the meantime, an honest chat with a trained pharmacist in a chemists.