I do not know but, I just wanted to tell you how cute your baby is!!!
I know a friend of mine said Cornish are hard to sex like a normal bird b/c they are just big and most folks think big legs, big size it's a roo but, she said unfortunately you can not do that with these babies
I know that doesnt help you but, I figured If I chat with you it will bump and keep it going for you too besides telling you what a cutie pie you have
I'd say roo. It looks like he's getting a solid black breast. A dark cornish pullet/hen should have more lacing on their breasts and evenly over the whole body. The comb is a bit red and big to be a pullet too.