I agree with you (and him) that "science can tell us what human activities foster and correlate with human prosperity and happiness." However, that presupposes

I agree with you (and him) that "science can tell us what human activities foster and correlate with human prosperity and happiness." However, that presupposes that "human prosperity and happiness" is something that we should consider valuable. This is (almost) a classic utilitarian POV. But what if we don't all agree that utilitarianism is what we should be after? What if you're a Kantian? Or an Egoist? Or an emotivist/expressivist? Harris sells us utilitarianism and utilitarianism alone. His views would be much more palatable if he distinguished between the moral theory he is espousing and his argument that science can help us interpret/apply the theory.