Re: 6 yr. old smearing feces
can you bring him to a child psychologist? they usually have some 'tricks' to get kids to talk; if it isn't sexual abuse it may be something else, so you probably don't want to guide him in his answers to your questions.
if you do start asking him questions, make sure they are open ended, non-guiding, and non threatening/forceful. maybe you shoudl pick up a book on communication with children? i'm sure there are lots out there.
but at the same time, if you do get him to talk....you must pre-plan your reactions. what if he said he was sexually abused...will you start balling and screaming? how would that make him feel? would you pretend it never happened (i guess you wouldn't you sound concerned)?...what if he comes out with something totally out of the blue (ie like he's hearing voices in his head)...my point is you have to gauge how your reactions will be because they can effect how he perceives himself or the situation . be prepared for anything and everything if you insist on having a discusion with him.
follow your gut instinct 9the sexual abuse may be wrong, but if something is telling you that his behaviour is not right, its probably true).
kids do fight though and have major resentment issues (maybe thats the only problem). do you get along with his mother? it sounds like you do not approve of some of her parenting behaviours. just because her friends are wierdos doesn't mean they are hurting her son. make sure you are not confounding these two issues together to the point that you will not accept any other explanations.
i still think a psychologist is the way to go (though you will need his mother's and his consent).
good luck, keep us posted.