It may or may not be serious. He may be retaining water, this will cause swelling. Did he get an insect bite, sometimes this will cause swelling as well.
I would suggest that if it gets worse or does not go away to see his doctor, it is probably nothing, but, with diabetes, you can't be too careful.
Everybody over forty has ocasional swelling in one or both legs. If the swelling persists, it's time to see a physician. Persistance should be defined as lasting longer than two weeks, or recurring more than once a month. As SassyLassie stated, there are many possible causes for fluid retention, some serious, and some not so serious. This is one of those situations where you really need to see an M.D. in person and get a good general work-up. Salt reduction in the diet is usually the first step in treatment, and usually leg elevation and an elastic stocking are suggested. One of the old-time treatments was use of the traditional rocking chair.