By Linda Sieg

TOKYO, Oct 18 (Reuters) - When Japan's youngest prince, Hisahito, visited Bhutan in August on his first overseas trip just months after his uncle Naruhito became emperor, his trip was regarded as the debut of a future monarch on the world stage.

Greeting his hosts in traditional "hakama" kimono and trying his hand at archery, the visit was rare public exposure for the boy on whose shoulders the future of the monarchy rests.

Emperor Naruhito, 59, who became monarch on May 1 following the abdication of his father, Akihito, will proclaim his enthronement in an Oct. 22 ceremony before foreign and domestic dignitaries.

Japan only allows males to ascend the ancient Chrysanthemum Throne and changes to the succession law are anathema to conservatives backing Prime Minister Shinzo Abe.

Hisahito, 13, the lone royal male in his generation, is second in line to the throne after his father Crown Prince Akishino, 53, the emperor's younger brother.

"Under the current rules of succession, Prince Hisahito ... will eventually bear the entire burden of perpetuating the imperial family," the Asahi newspaper said in an editorial this year.

"The pressure this prince would eventually come under is too formidable to contemplate."

Hisahito's birth in 2006 was seen as a miracle by conservatives eager to preserve the males-only succession.

No imperial males had been born since 1965 and after eight years of marriage, the emperor's wife, Masako, gave birth to a girl, Princess Aiko, spurring moves to revise the succession law and let women inherit and pass on the throne.

But Hisahito's birth put those moves on hold. "Conservatives felt that the will of heaven had been revealed," said Hidehiko Kasahara, a professor of political science at Keio University.

ROYAL ROLE, IMPERIAL SUCCESSION

Now, some experts and media are wondering whether Hisahito is being properly groomed for the future.

"It is important to have him realise that he is in a position to inherit the throne when interacting with people, and to keep them in mind, from an early age," Kasahara said.

Japan's post-World War Two constitution gives the emperor no political authority, and designates him the "symbol of the State and of the unity of the people".

Hisahito is attending a junior high school affiliated to Ochanomizu University, making him the first imperial family member since the war to study outside the Gakushuin Junior High private school.

Unlike his grandfather, Akihito, who carved out an active role as a symbol of peace, democracy and reconciliation with victims of Japan's wartime aggression, Hisahito has no special mentor to help him prepare for his future kingship.

Akihito was mentored by Shinzo Koizumi, a former president of Keio University, among others, and then became the role model for his son, Naruhito, scholars say.

"It is necessary to have someone who can determine with him what is appropriate for a 21st century monarch," said Naotaka Kimizuka, an expert in European monarchies at Kanto Gakuin University.

"But it is not clear to what extent Crown Prince Akishino or the Imperial Household Agency is seriously considering that."

Whether Hisahito bears the full responsibility for continuing the imperial line is as yet unclear.

When parliament passed a special law allowing Akihito to abdicate in 2017, it adopted a non-binding resolution asking the government to consider how to ensure a stable succession.

One option is to allow females, including Aiko and Hisahito's two elder sisters, to retain their imperial family status after marriage and inherit or pass the throne to their children, which surveys show most ordinary Japanese favour.

Conservatives want to revive junior royal branches stripped of imperial status after the war.

Abe, though, is unlikely to want any thorny discussions. "They want to put off debate as much as possible," Kasahara said.

(Reporting by Linda Sieg; Editing by Karishma Singh)