13 seems old, even for emotional bed-wetting which my 5 year old is sometimes prone to. I am sure it is linked, I just think it is indicative of how great his emotional upheaval is. I doubt it is worry about being at your house, but more linked with his emotional issues from his mom's house. I would ask the mom if he has been wetting the bed at her house and find out how often. I would also talk about getting him some therapy, given all that he has been through, even without the manifestation of his inner emotions with bed-wetting, I think it would help with the transition and addressing residual abandonment issues (and whatever caused CPS to pull him in the first place).
I think an open conversation with your nephew is appropriate too. I always believe in open and honest lines of communication with kids. Sit him down for a serious talk, let him know you care, and you want to find out how things are at his mom's house. You know it has to be rough after so many years. Ask him what he likes best, what he dislikes most. Remind him he can always be honest with you and you'll always have his best interest at heart. If anything he says rings warning bells, ask him if he would prefer to stay at his mom's or come back to you guys, let him know he is always welcome but in a non-pressure way so he doesn't have the added guilt of letting someone down.