Constipation in kids. Be done for this?

My son is having constipation from 1 week, he actually passed hard stool for 2 days and there after passing soft stool(not diarrhea), but having lot of pain while passing it. For now we are using stool softener and giving rich fiber diet. HE ACTUALLY HAD SAME PROBLEM COUPLE OF YEARS BACK.Can someone please answer below questions 1. Why this problem recurred after two years 2. Why my son is feeling pain while passing soft stool 3.He always complain mild stomach pain after eating any meal. This pain lasts for few minutes.Why does he complain and what should be done for this?? 4. When will this pain go away while passing stool. 5. What should be done in order not to reoccur this problem. Please help me and my kids on this. Thanks in Advance