I know Kennedy discusses a lot of what I'm going to say, but I had these thoughts before I read his chapter. So, I'm going to lead with what my original thought:
I was really impressed with the way Aristotle addresses both the nomos and the physis crowd throughout 'On Rhetoric'. As I read, I consciously switched back and forth between a Gorgian and Platonic perspective. Here's what I found:
Aristotle makes some concessions to the Sophists by implying at various points that man is the measure of all things. For example, at 1359b he states that "we limit our consideration to the point of discovering what is possible or impossible for us to do." He is addressing human actions in deliberative rhetoric here, and he places heavy emphasis on the possible. Essentially, I see Aristotle arguing that man, in the forum of customs - the deliberative body - is the arbiter of his own destiny and that the possible and impossible are determined by man. I suppose this read hinges on a particular interpretation of whatever Greek word is used for 'discovering', but I think the end of that paragraph is instructive as to which use Aristotle intended.
He says, "In so far as someone tries to make dialectic or rhetoric not just mental faculties but sciences, he unwittingly obscures their nature by the change, reconstructing them as forms of knowledge of certain underlying facts, rather than only of speech" (1359b.6). I see him here saying that logos is not fixed and eternal but growing and changeable. If logos is used in the deliberative assembly, and that assembly determines the possible with the malleable medium, their conclusions are also malleable (and so constructed), not eternal and formal.
He caters to the Platonists on the topic of justice when he says that there is the law of gods and the law of man and that the laws of god are clearly higher (1373b). He defers to the universal concept of fairness when the laws are not sufficient to enact what is universally equitable (1375a). It seems that, for Aristotle, the forms are present, but they are encoded in the Gordian knot of human judgment.