That looks like a hen to me. BR roos have alot more white and the black is a lighter color, and the hens are more black with a little bit of white. Also, you said he is 12-15 weeks, my 9 week old BR roo has a comb the same size, if not bigger, and has alot more white on him. You can also tell if he is a roo or hen by the way he acts. I knew mine was a roo before any real physical features showed up because he was always the first to get anything and he always came after me when I went to feed or water them.