Yes, this is very odd. Begin with the Dr., then go on as recommended. An excellent neurologist and/or psychiatrist may be needed. Think about when the behavior began, what the current events in the family and at school were at that time, and do go to the Dr. Hurry. A lot of things show up at his age that may not have been visible before. Toileting/ bowel issues indicate a huge regression. Be watchful of the 13-year-old's behavior with the 19-month-old. Do not leave them alone together.
I have taught school for many years, most of them at ages 12-21. I can tell you that the boy is not being 'lazy'- no thirteen-year-old messes his pants without a major psysical or psychological reason. I can see one time, if he had an illness. But three is beyond strange. Is he being allowed to go to the bathroom at home? and at school? Or is there some crazy out there trying to 'make a man' of the boy, and making him crazy? Is this possibly revenge for something that has been done to him?
Your instincts are right on the money. Go with them. This behavior is a cry for help, and should not be ignored or made to go away.
My thoughts are with you and your family - S.