Well...to be serious (at least as serious as one can get when talking about a Disney/Square Enix video game) I think Sora would already know and wouldn't be so shocked. Sora seems to usually know the answers to questions subconsciously. Like with Roxas. And I'm sure if Sora went into a dream state right now, he would know who Ven is.
But if they make Sora naive again, he most like will go "WAT? WHY DO YOU LOOK LIKE ME! IGNORE THE FACT THAT I SHARE MY HEART WITH VEN AND KNOW THAT YOU ARE HIS DARKNESS. BUT WHY MEEEEEE?"
Or he will be kinda like "o_o" and have a memory moment inside of his heart.
But I mostly see him being very calm about it.
He might be calmer now, but he's still an idiot. I mean, he was the last person to figure out who Roxas was and put 2 and 2 together...well, maybe it took Goofy a bit longer, but that's understandable. Sora even had to ask for help...
Sora: "Hey! Why's everyone been calling me Roxas?"
Riku: "Because, Sora. He's your Nobody."
Sora: "My Nobody!? But I never turned into a--ohh, that's right." *lightbulb!*
And also, he's got a ton of look-alikes/clones/Nobodies that he technically shares some part of himself with. He's like an organ donor!
Sora (Himself.)
AntiSora from KH1 Neverland
Roxas
Xion
Ventus
Vanitas
Data Sora
Namine...? (What exactly IS she anyway? She appeared in Castle Oblivion, which is the Land of Departure where Ventus sleeps, because of Kairi's heart coming from Sora's body, but returns to Kairi at the end of KH2. I smell a plothole.)
Bottom line? Everyone in KH is sorabasically, even if they have demtits.