C.S.
My son had this problem but his was becasue it was hurting to poop. He had really hard poops! They gave him miralx which you can now buy over-the-counter. its just makes it softer not a laxative. ask him if it hurts adn we had to actually collect it so you may wanna see what happens when he does actually decide to go. my son could hold it for days! it scared me becuase he would stop eating cus he was backed up.
or you could try putting a diper on maybe he doesnt wanna do it in the potty. at least it will "clear" him out! i actually have put vasaline in his "area" to help lube up the hold beacuse the dr told me it will stimulate the process! hope this all helps!