Could he perhaps benefit from a substitute? I like to manipulate things in my hands, but I grew up with modeling clay available. I'd make stuff out of the modeling clay or kneed it. I also liked silly putty.
That may be a possiblity. See if that would help fufill the need to mold something in ones hand without using fecal mater.
The management of the behavior should be the priority, and the best approach is a simple disciplinary system. It would likely involve an immediate time out period. If the behavior is intermittent, rather than continuous, it is likely a response to stress. Howver, it may not be obvious what exactly the nature of the stress is with an autistic child. Needless to say, very close supervision is required. There's no point in engaging in much discussion about it. Rather, very clear, simple limit setting and dsicipline should be applied.