My son is 14 years old and he has constipation for 3 days and he can pass gas and poop but they are pencil like. after eating some dried prunes, he started having watery diarrhea. what to do next?

Back off on the prunes. Encourage him to drink adequate amounts of water daily and eat fiber from fruits and vegetables daily. Regular exercise should help. Occasional pencil thin stools are not of concern. It doesn't appear that he has a serious gi issue, but if his bowel pattern doesn't return to normal, have him evaluated by his physician or a gastroenterologist.