I see I think he is waiting too long this isnt unusual its a kind of laziness,but I am sure he doesnt do it on purpose, explain to him that he is waiting too long ,as as soon as he feels the need to go he should go sit in there and wait, he could be a bit constipated so check his diet out.Sometimes they get so engrossed in what they are doing they ignore the feeling.
He only does it at home, it is just the beginning of needing to go. I don't believe anything has happened in our life to do this. He is an only child, seems pretty happy.
Does he do this anywhere else ,school and other familys house?Has there been an upset recently in his life,Are they fukll Bowel movements or just staining and the begining of needing to Go, could be he doesnt get there in time, check out what else is going on.