My son (25) has one tooth (lower front) that the gum has receded a lot in a very short time. The gum still looks pink. I have never seen anything like it before and it wasn't there at his last Dental visit. Any clue what could have caused this?

By any chance is your son's tongue pierced?

There are a few causes of gingival (gum) recession. As flossie pointed out, if it is in the lower front, it could be from something rubbing on it (peirced lip, a habit of chewing/sucking on a toothpick or something). Braces can push teeth in a position where the gums can't cover them. Bad hygiene (not flossing/brushing enough), TOO MUCH hard brushing on the gums, chewing tobacco, smoking, or crowded teeth can lead to it as well. Sometimes there is no apparent cause. If you have the funds, you can go to a periodontist to get a graft, but if he has a minimum (2 mm) of hard pink tissue, and the tooth is not wiggly, AND he has good hygiene, goes for cleaning, and does not brush too hard, then it should be stable and fine.

No pierced tongue or lip. He does smoke and I can't be positive about the hygene, my guess would be not that great.