According to your theory, the masked man is an Uchiha, who has the power to mind control people with the Sharingan, and is controlling that kid who isn't an Uchiha, to plot vengeance against Sasuke.
Why would that masked man who has the power to control someone with the Sharingan, would feel the necessity to mind control a kid who isn't even an Uchiha to plot something against Sasuke? What could possibly be so special about that kid? This whole relevance to the Uchiha clan is a dead giveaway that the child is an Uchiha. A non-Uchiha kid, who's just controlled by the Sharingan, would have no concern with the Uchiha clan or hatred against Sasuke.
BTW, there's no evidence from anywhere that a mind controlling genjutsu cast upon someone manifests a pair of Sharingan. If that would be the case, then everyone would immediately know Yagura was being controlled by an Uchiha because he would have a reflection of the Sharingan, but nobody guessed it. Why? Because it was just a mind control genjutsu, and genjutsus don't have this effect. This isn't a summoning like the case with Kurama.
In addition to that, someone who is mind controlled doesn't talk to the jutsu caster like that. Because you can't think on your own since your mind is being controlled by somebody else, so the person being mind controlled will only say and do whatever the jutsu caster wishes. Since he is speaking of his own consent, he's not being mind controlled. So even by pure logic, this theory isn't viable.
(Based on the bolded points which have been explained above) He's definitely not mind controlled, which is even more affirmative, if you refer to Yagura's case. And since he is concerned about that Uchihas, it is proved that the kid is an Uchiha.
