In KH1 Sora released his own heart in order to save Kairi, whose heart was in Sora. Because of this, Sora turned into a Heartless, that's obvious.
But why was it a Shadow? A Shadow is the lowest Heartless in the KH universe, I mean there exist literally millions or even billions of them, because every ordinary guy who loses his heart to the darkness turns into a Shadow.
But Sora is no ordinary boy (even though Nomura tries to convince us otherwise), he's a Keyblade wielder and he has a strong heart, or else Roxas wouldn't exist, that's proof enough that his heart is strong.
For example, Xehanort's Heartless is the only one with a human form, because he had such a strong will and heart. I'm not saying that Sora's Heartless should have a human form too, since he was not nearly as strong as Xehanort at that moment, but it should be at least a stronger Heartless.
So why a Shadow?