One way to convince him to sit is to set a timer. Let's say you want him to sit for 5 minutes—set your timer for just that long, particularly a timer that he can hear as well. That way, it doesn't feel like he has to sit on the toilet until you say so. Another way is to give him something he can do while he sits, like reading a book or using a tablet. This special treat can be a way to not only make the time go by, but it can distract and relax his body enough that he passes bowel movements.