After Squealer is found near the barn with paint everywhere, obviously drunk, the animals wonder about the commandment forbidding alcohol. But when they check it, it has been amended to say that animals cannot drink to excess.
Benjamin is the only animal to realize that the commandments change whenever Squealer has an "incident".
He refused to read the sixth commandment at the beginning of this chapter because he did not think it was any of his business and not his place.
But it is apparent that Benjamin has known all along what is really happening on Animal Farm, and has just made the decision to not say anything.