Hun Manet was appointed Cambodia's new leader by the king on Monday, after having effectively been given the post from his father, who ruled for nearly four decades.
Days after a landslide victory in July's election, Hun Sen — one of the world's longest-serving leaders — announced he was stepping down as prime minister and handing power to his eldest son.
The polls were widely decried as a sham after the main opposition challenger, the Candlelight Party, was barred from running over a technicality, with the ruling Cambodian People's Party winning all but five seats in the 125-member lower house.
On Monday, following a request from Hun Sen, King Norodom Sihamoni issued a royal decree stating that he "appoints Dr. Hun Manet as the prime minister of the Kingdom of Cambodia for the 7th mandate of the parliament."
To officially become the country's next leader, the 45-year-old and his cabinet must win a confidence vote in parliament set for Aug. 22.
The lower house is overwhelmingly stacked with members loyal to Hun Sen and he has pursued a ruthless campaign against any opposition, banning parties and forcing adversaries to flee abroad.
Longtime rival Sam Rainsy — self-exiled in France — said any hopes for more freedom under Hun Manet or that he might distance the country from major benefactor China were misguided.
"I think this is pure illusion," he told AFP ahead of the appointment. "You can change leaders many times but nothing will change because the system remains the same."
The incoming government will usher in a crop of young ministers, with some taking posts vacated by their fathers.
While insisting he would not interfere with his son's rule, Hun Sen has also promised Cambodians that he will continue to play a prominent role in politics.
"It is not the end yet," he said Monday following the appointment, adding he would continue serving in other positions until at least 2033.
After stepping down, Hun Sen will become president of the Senate early next year and acting head of state when the king is overseas.
Despite being groomed for years, the eldest son of Cambodia's iron-fisted ruler remains untested in the political arena and relatively unknown beyond it, according to Sebastian Strangio, author of "Hun Sen's Cambodia."
"It is very hard to determine any distinctive national vision," he told AFP.
Hun Manet has an economics degree from England and graduated from US Military Academy West Point, but there are few signs he would bring his country closer to Western powers.
"I have not seen any evidence that Manet has either the ability or the desire to push Cambodia in the direction of significant reform," Strangio said.
He said Hun Manet's decisions would be "dictated by" the country's rigid political system, requiring him to maintain the loyalty of powerbrokers in the political establishment, business elite, and security forces.
The 45-year-old Hun Manet is a member of the ruling party's powerful permanent committee and has been the Royal Cambodian Army's commander since 2018.
He has in the past met world leaders, including China's President Xi Jinping, Cambodia's main ally and a significant benefactor.
Following the announcement, Hun Manet posted a congratulatory song on social media.
Hun Sen has rejected international condemnation that last month's polls were neither free nor fair, saying the handover was done to avoid "bloodshed" should he die in office.
A former Khmer Rouge cadre, Hun Sen has run the kingdom since 1985.
He helped modernize a country ravaged by decades of civil war and genocide, although critics say his rule has also been marked by environmental destruction and entrenched graft.
Rights groups accuse Hun Sen of using the legal system to crush any opposition to his rule — jailing scores of activists, union leaders and lawmakers.
In March, opposition leader Kem Sokha was convicted of treason and sentenced to 27 years in prison, currently being served under house arrest.