I haven’t read Sam Harris, my only knowledge of him so far is this question and an incomplete reading of his Wikipedia entry. But he’s attempting to quantify “morality” without respect to various religions. This is, in fact, an essential task if we’re ever going to arrive at a truly secular government, which doesn’t attempt to define its policies in terms that ultimately stem from religion. (Though we try to separate church from state in the US by mandating that there shall be no state religion, most legislators, judges and presidents ultimately decide policy with a bible – or torah – on hand.)
I think your question attempts to simplify or reduce his arguments to a point of absurdity (whether that’s your intention or not).
But to attempt an answer to what you did ask, I don’t think that anyone can define another’s “happiness”, if that other person is conscious and capable of thinking on his own. Sam Harris can’t “measure” your happiness or mine, but it’s up to us as individuals to attempt to measure our expected / anticipated outcomes against proposed policies to help determine “aggregate happiness” – if such a term isn’t too absurd on its face – and from that to help steer public policy. (I think this has to be done within a framework of respect to republican ideals and respect for individual freedoms; though it might “increase aggregate happiness” to kill Warren Buffett and pass out pieces of his body for us all to eat, that would be a false action, since it would obviously violate Mr. Buffett’s individual right to enjoy his own life.)
So, for example, I’m all in favor of operating on a person with acute appendicitis to alleviate the pain and remove the diseased organ. Does that mean that we should all have our appendixes removed to attempt to prevent “future unhappiness”? I don’t think so, but I’ve never had appendicitis; maybe I’m not even qualified to say “no” to that.