I think he heard some philosophy stuff out of context or over simplified, and was knee deep in inferences when he tweeted that. Something like:

• The only thing we know with certainty that we exist.

• So we don't know if anything, including what we perceive, is real.

• If our perception could be fake, we can't even know that our sensory organs, like eyes, are real.

• However, in my perceived life, I can look at myself in a mirror, and see my own body and my own eyes.

• If my perception isn't real, and my eyes aren't real, how can I see them in that mirror?

• If our eyes aren't real, we wouldn’t be able to see them in a mirror for... some reason.

• But my name is Jaden Smith, and I can't understand that a fake reality could trick me into thinking I see my own eyes, so I think being able to see my eyes in a mirror means my eyes must be real, the mirror must be real, and the physical world exists with certainty. I disproved solipsism guys!

In all seriousness, I don't think he believed his eyes weren't real, he was asking a rhetorical question that he thought proved something deep. However, because he was a dumb teenager, he thought he could figure out something no one in history ever considered, and was thus making some grand contribution to society.