I was wondering if anyone could help with the Q or even explaining what/why he rejects a deductive ethical science? thanks so much!
Excellence of character and eudaimonia aren't deductive, they're virtue-based: it's about phronesis, which is itself about doing and not about knowing. He thought that virtue was habitual, not abstractly knowledge-based (and so not sophia). This is why he rejects deductive ethical reasoning. It's not enough to 'know', and someone that 'does' might not really 'know' why they do it (because it is habitual, how they were raised etc).
But it's not even that Aristotle simply combined Plato's view of ethical truth with virtue - Aristotle also rejected the idea that there could be one form of good which covered all things. He claimed that what it means for one thing (or profession) to be good is vastly different from what it means for another thing (or profession) to be good. Since the situations of life are so diverse in terms of what things occur in them and in what way they occur, you cannot have absolutist rules which cover all situations.
So, the best we can do in ethics is develop our virtue so we have the will to do the right thing and practical wisdom so we can figure out what the right thing to do in each particular situation is.