Phillip shows extraordinary bravery on a number of occasions in the story. First of all, he acts bravely when his ship is torpedoed. Many adults in his situation, let alone children, would've panicked, but not Phillip. Though now blinded, Phillip shows a consistent determination to overcome the challenges presented by his disability. And that in itself is brave.
When Phillip and Timothy fetch up on the cay, the young lad initially shows himself unwilling to get with the program and learn how to survive in such harsh conditions. More than anything else, Phillip is held back by a debilitating fear, as can be seen when he refuses to climb a coconut tree. Yet one day Phillip manages to put his fears behind him and climb the tree all by himself. This is no mean feat for someone in his position.
But the greatest example of Phillip's courage is his ability to survive by himself after Timothy passes away. Timothy is responsible for giving Phillip the necessary skills and tools for survival, but Phillip still has to put what he's learned into practice. That he's able to do so, despite his disability, and despite being all alone, is a testament to the young boy's seemingly inexhaustible reserves of courage.