You're kind of being weird.
How in the world would his teacher know that he doesn't wipe his butt?
You have a family therapist, who you've taken this distressing situation to, ????, and now you're all worked up that his teacher will call you.
She won't know, and won't call. Rest assured. I had an 8 year old who didn't wipe his butt either. Eventually, they do.
He's may be angry with both of you. Ask him if he's angry. Is he generally lazy about responsibilities, too? What is his mood like? How is his effort in outside of the home, with school work? You can try an incentive (star chart for good behavior). Make sure he knows what it means to do a good job, by showing him exactly what to do. If that doesn't work, you might want to tell him (in a firm but not punishing way) that for a time you will be checking him every time he makes a bm and if he's not clean you will be sending him back to do it again. If you need to stand over him while he does it for a short time span, do it until he shows you he is trying. If he doesn't try, tell him he's not be allowed to watch television, play with toys, go on the computer, have special treats or anything else that is fun until you see he is making more of an effort. I know it sounds harsh, but you want to deal with this aggressively at his age. Also, I would suggest leaning heavily on him about other responsibilities, such as cleaning up his toys and helping out around the house to try to influence him in areas other than toileting with the hope that it will carry over to cleaning himself. How does that sound?
You said that the family therapist "thinks that maybe it's because he's under stress due to our recent separation". I think the therapist could be correct - this behaviour is common to children suffering from anxiety or extreme stress. I doubt if your son is doing this on purpose - he probably does not know why he is unable to clean himself rather than this being a manipulative or controlling behaviour. What did the therapist suggest?