Yes, sociology is and can be scientific. Kemper is too pessimistic. There are two points at which I disagree with Kemper's analysis.
Different theories deal with different social processes.
And, even in developed areas, there may be two or more reasonable competing theories. That we be explicit about our theories and that we try to improve our theories does not require that we all have the same theories. Second, the kind of theories and predictions Kemper suggests are too concrete. Abstraction is one of the keys to scientific development. So, I share Professor Kemper's concern for cumulative knowledge in sociology.