Please help! Our 11 yr old foster child won't wipe and now I found a terd in the shower. What is up with this?

Our nephew has lived with us since Aug. He has made a lot of progress since he has been here, but he still will not wipe his backside after going #2. I have found feces all over my laundry after it has gone through the wash. Today I found a terd in the shower on the ledge where the shampoo sits. What is up with this? Is this some psychological issue dealing to his post traumatic syndrome? Any suggestions would be appreciated. I have 3 other children and have never delt with anything like this before.