is he holding an apple?

yes he is

lol. a GREEN apple, go figure

Why is he holding that apple?

i like apples

He's holding a green apple because it's reminicint of POA where he was eating that green apple right before he got hurt by Buckbeak [when he was shoving everyone around and smirking]. It's showing that his prespective has changed. He's seeing the inside of the apple, that there's something more than the "green" skin of a Slytherin. It might be green on the outside, but it could also be white on the inside. He's looking at the white part, which is glowing like the lit parts of the ROR & Draco. It'sBeautiful!

Um.. no. Lol. He puts the apple inside the Vanishing Cabinet to see if it works and when the apple comes back, it has a bite taken out of it, therefore telling him the cabinet works.

Thank you ever so much for the spoiler... now I know that in the movie when he puts the apple in the cabinet it's going to work. I decide not to watch these extra things on purpose so as not to loose the magic of the whole film put together.

I think ticktock is right to an extent. they used a green apple to draw the connection from POA. But maybe not the rest. Jess is also correct he used it to test the VC.