Periodic stomach cramping/pain lasting about 6 hours

I can't figure out what the hay is going on with my 18-year-old son.  He's quite healthy, other than the fact that he doesn't eat enough vegetables.  There is no overall pattern to this--it may happen once and then a week later, and then not for a month, but it does seem to happen only in the evening and has been going on since about 6 months ago.  His cramping/pain is in the middle of his abdomen, just under his navel.  He has no fever, and it doesn't seem to be brought on by any particular food.  I do plan on taking him to his doctor, but she is pretty booked up and it may be a few weeks.  I'm trying to see what the possibilities are so that I'll know if we should wait.

[ggreg]

If it's gas, an ordinary alka seltzer should help him immediately.  Gas can hurt a whole lot, double a person over even.  If it's occasional constipation or diarrhea, he can eat more whole grains and it'll do the same thing as fiber in the vegies he doesn't like.  Sometimes both gas and indigestion can be caused by not eating regular meals through the day, followed by eating a huge meal in the evening.  Also, stress can cause his symptoms, so exercising regularly will release tension.  And always plenty of water is good for the digestion.