Awww, they're so cute when they're little! For most aros, you can tell the sex when they get older. Aros are mouthbrooders, meaning the male carries the eggs (or fry, I can't remember). So his top lip protrudes above his bottom, to give him room to carry them. The female's lips meet up, with no protrusion. Hopefully, this is the case with this type (and if it's not, I'm sure someone will correct me)! Good luck with him -- he's a cutie!