I heard about such things and it was cured just as time passed, but I am not sure whether it is the best recipe. Doctors say that about 10% of children at this age are still enuretic at night. But I haven't heard about any other recommendations other than to limit the fluid intake during the couple of hours before going sleep.
Also we have friends and their 10 years old daughter still sometimes has such night problems. But in her case doctors says it's rather a psychological problem - he believes she requires more attention and love from parents and this is the way for her to gain it - at least in some form. It doesn't seem to be your case so probably you just have to wait and your son will outgrow it
