I studied biology at college, but I'm no expert on this subject. Genetic engineering ALWAYS has to be done at the embryonic single celled stage. Once the cells have divided it simply becomes impossible to alter the gene in every cell responsible for expressing that gene and producing 5 alpha reductase. Again, I'm not sure what role DHT plays in male develpment, the kid would still have testosterone, maybe that would be enough for normal development?
I'm pretty certain that multiple genes are responsible for male pattern baldness, possible at different loci, but the human genome has already been mapped, it should be a realistic challenge, but still, identifying the responsible genes across 46 chromosomes, each with tens of thousands (or millions?) of genes would be no small task!