Is he getting enough nutrients and developing properly? Is he drinking lots of juice? What about eating cookies or foods with a high sugar content?
My daughter had the same problem from the time she was born, it used to leak out of her diapers. Since she was breastfed I expected it at first, but as it continued after solids were introduced the doctor advised diluting her apple juice half and half with water, then finally we had to stop giving it to her altogether. We also had to stop allowing her to eat anything with refined sugar, so lots of breakfast cereals were out. She was always on the thin side so I was worried, but he said since there were no allergies, she was developing properly, etc., to ride it out, and that he would refer us to a gastroenterologist if anything changed.
It seems she was about 2 before things began to firm up gradually, and she did eventually outgrow the problem.