In the story the boy finds the entrance to a tomb in a hill, where he finds the weapons of a dead feline warrior. Putting on the helmet enhances his senses. At the end of the story he is medically examined by a computer and restored to perfect health, before the tomb is removed from earth to prevent it being found again.
It's not a short story, but this bears a remarkable resemblance to Robert Westall's young adult novel "Urn Burial". The boy, the tomb, the feline warrior, and the medical examination are all definitely present in this book.
You can find a more in-depth exploration of the book's plot and themes here.