MY 4 Year Old Grandson Is Having Great Difficulty With

MY 4 year old grandson is having great difficulty with constipation. He may go for 2 weeks without a bowel movement. Parents have increased fiber, fluids and he gets plenty of exercise. Tried Miralax OVTC with some other remedies. When he does go it is very painful and large hard stools, probably the reason for holding it in. Even bribed going with rewards... suggestions????