His website. Download The Terrible, Horrible, No Good, Very Bad Truth about Morality and What to Do About it (hosted here, it is also available from his website). In it you will find: a defense of antirealism, an explanation of morality in evolutionary terms, an explanation of moral intuitions in neuroscientific terms, a discussion about what to do given that antirealism is true, his ideas about doing it. It's not analytic enough making some parts a bit unclear (for instance, he talks about an analogy holding but only imperfectly cf. p. 254). I think the argument in favor of antirealism is unconvincing but worth reading. Perhaps something similar works. If you are a Hume fan, then you'd want to read this as a whole chapter is dedicated to how right Hume was about the source morality (the sentiment). I enjoyed reading it. 377 pages.