sorry to hear you are experincing dificulties with your wife son, i have a son who is 15 and will be 16 in september, you might not understand this but my son is adhd he also ,esses himself and sometimes sits in his on mess, there is a medical thing for this i have use of a adhd were other pppl talk about similer things,
my son up to the age of 6 would do this several times a day , we now have him in a routin that helps us and him,
he has his meds as i wake him first thing in the morning he then sits and eats his breakfast, then he takes nintendo ds and sits on the toilet for at least half an hour making sure i tell him that the time is and giving him ten mins before he is due to get dressed reminding him after 5mins, if he misses this routine or does not take his adhd medication he will not be able to sit and go to the toilet, im not telling you this is why your step son is like this but i understand why this is frustrating, my son still wont tell me he has done this he trys to hide it from us,you are welcome to p.m any time