Is Sora a Keyblade master

Yes, Sora is a chosen keyblade master. He is the main Keyblade wielder, the second being King Mickey. Sora was chosen to be the keyblade master because of his powerful heart and strength.

SPOILER******************
Terra wanted Riku to be the keyblade master, but Sora's heart was more powerful, thus Sora was able to combine his heart with Ven, allowing him to wield any keyblade.