Soiled pants

My son is 12 was easily potty trained, but for a while now he has been soiling his pants.  He has always had trouble going to the bathroom.  He says that he does not know when he needs to go.  I believe this because even when I can smell it he will argue with me and try to prove that he has not gone to the bathroom in his pants.  It is not usually a lot.  I do not know what to do.  Do you have any suggestions?