Hi, I'd like to help you with your question.
Your boyfriend's son sounds like he may have some antisocial personality disorder traits. The usual symptoms include hurting and/or animals, bedwetting, and fire setting. It starts off as a conduct disorder in childhood and usually peaks at late adolescence or early adulthood. He could also have oppositional defiant disorder as well. This at least needs to be ruled out.
The child with conduct disorder is often described as witty, charming, he/she lies and steals, manipulates others, does not respect other people's feelings, and does not show guilt or remorse. Since your boyfriend's son seems to show some remorse, he may only have traits of this disorder.
There is no known cause of anti social personalty disorder/conduct disorder, but it is thought that childhood abuse or neglect, genetic factors or parents who are alcoholic can contribute to the cause.
Treatment is therapy, but it is difficult (but not impossible) since the person with the disorder does not want treatment. It is good that your boyfriend has his son starting treatment soon. It may have a good effect the earlier it is started.
Usually, the best advice in dealing with someone with these types of symptoms is to stay away from them as much as possible. But since this child lives with you that is difficult. You may want to consider asking your boyfriend to move out with his son temporarily until he has a chance to start therapy and see what the therapist says. Or, you can try to protect yourself the best you can by keeping a very close eye on your boyfriend's son. Don't let pets or children be alone with him. Lock up your valuables and warn others to come to you or your boyfriend with anything they see or hear the child doing that concerns them. Also, participate in the therapy by expressing your concerns to the counselor and asking for help in coping with the situation.
Here are some resources to help you as well:
High Risk: Children Without A Conscience by Ken Magid
The Challenging Child: Understanding, Raising, and Enjoying the Five "Difficult" Types of Children by Stanley I. Greenspan and Jacqueline Salmon
The Defiant Child: A Parent's Guide to Oppositional Defiant Disorder - Paperback by Douglas Riley
You can find these books on Amazon.com or your local library may have them for you.
I hope this helps,