Mysterious boy appears to Locke & Desmond, Locke gets agitated

Sayid leads Locke to where he has Desmond tied to a tree. Locke unties Desmond after Desmond says he has nowhere to run to. Desmond says that he doesn't know why Charles Widmore brought him back to the island, but mentions that they threw him into a wood shack blasted him with a massive amount of electromagnetism. When Locke asks how he knew it was electromagnetism, Desmond says that he knows from experience.

Locke clearly becomes unsettled by how unafraid and calm Desmond is. When Locke asks Desmond if he knows who he is, Desmond says "Of course. You're John Locke." Locke then sends Sayid back to camp and tells Desmond he wants to show him something.

Later, as Locke and Desmon are walking through the jungle, Locke tells Desmond that the island seems to have it out for him, but Desmond replies that the island has it in for all of them.

Suddenly, the mysterious boy appears in the jungle. It's the same boy from the episode "Recon," but here, he appears slightly older and has brown hair instead of blonde. Locke gets agitated by his appearance and tells Demond to ignore him. When Desmond asks who the boy is and if he knows him, Locke repeats his order to ignore him, but this time he's angrier. The boy smiles slyly, and runs off into the jungle.

So is this young boy Jacob, and if so, what about his appearance is causing Locke to lose his cool? He has seemed the most rattled the two times this boy has appeared.