Hi, I'd like to help you with your problem.
You said that your son has had encopresis for about 5 years. I am assuming that any medical reasons for his problem has already been explored. Therefore, you are left to conclude that his issues are psychological in nature.
The first thing I noticed was that your son's encopresis started within a year after your divorce. Since I don't know what your son's therapists have worked on with him, I am going to say my feeling is that your son is reacting to the stress he feels as a result of the divorce and the conflict with his father's new wife. Psychologically, it appears that your son may be trying to exert control over his bowels because he cannot control the situation going on around him. Being a male is also a factor. It occurs more often in males than females.
I highly recommend working with your son's doctor, if you have not already, and make sure all of his physical treatments, such as a high fiber diet and an increase in liquids are being utilized.
Once he is attended to physically, the psychological aspects of what he is going through need addressed. You said that he is seeing a therapist right now and that is great. But has the original problem that caused the encopresis been addressed? You and your ex husband must find a way, for the sake of your son (and daughter too) to get along or at least be civil enough to one another to help your son feel safer and less stressed. I know it's easier said than done, particularly if the other parent is difficult and refuses to help. But if you approach him in a manner that is totally focused on helping your son, he may consider it.
See if you both would be able to meet with a therapist and have the reason and subsequent meetings be totally about your son and his issue. No accusing, no discussing any other issues. The goal here is to make your son feel better and recover. It takes a lot of strength to put aside your differences but if it's for the sake of your child, it is well worth it. Your children must come first and with help and practice, I believe this can be accomplished and your son will recover.
I hope this helped you,