What I would suggest is to have him sit on the toilet for a set amount of time every day at the same time every day. After a meal works well. Have him stay there for 10-15 min. If he produces great and if he doesnt, thats ok too. after a while (could be days or weeks) the childs body will usually start to become trained to poop at that time. Make sure that its a non-pressure situation, what we are trying to do is to get the childs body to relax. It does take time but it will work.