My guess, from this picture, would be that it is a roo. Those red feathers are pretty telling, but you can also see the beginning of development of wattles, and the two together almost guarantee that you have a roo. I know that you don't want to feed a roo any longer than necessary if you don't plan on keeping him, but it would be worthwhile to raise a few and observe the changes until you are sure what you have. Over time you will be better able to recognize the "clues" that tell you what sex you have. Some young roos start crowing fairly young, (I had one Ameraucana that crowed in the brooder at about 3 days. Seriously! It just sounded like a little squeak, but he would answer the adult roosters crow.) while others don't start until sexually mature. Crowing, and the development of saddle feathers are definitive signs of a roo.