Have you taken him to a doctor? I think that you need to rule out there is not an underlying medical problem. I know it is not pleasant but try not to get mad at him or show him you are disgusted as this will make things worse and harm his self esteem. He is probably very embarassed. I hope you can get this sorted out soon.
www.eric.org.uk is a charity that deals with both soling and daytime/nightime wetting. It might be worth a look. Is he constipated? That can cause problems with soiling.
