Dr Chakriwat 'Than Aong' Vivacharawongse, the third son of His Majesty the King, has arrived in Thailand to join his brother, Vacharaesorn.
The two sons visited the kingdom's first hospital, the 135-year-old Siriraj Hospital, and paid their respects to an image of their late grandfather King Rama IX and their great-grandparents there on Sunday.
Dr Chakriwat, 40, arrived on Saturday following the visit of Vacharaesorn 'Than Aon' Vivacharawongse, 42, who was a surprise arrival last Sunday having spent almost 30 years away from the kingdom.
The two planned to leave for the United States together later.
His younger brother's arrival prompted Mr Vacharaesorn to postpone his departure on Sunday in favour of showing his brother around for a few days.
The brothers visited Ayutthaya province on Saturday. They also planned to take a skytrain and an electric boat trip.
Dr Chakriwat is also the founder of New York's Chakriwat Medical Information Center. He expressed his interest in giving lectures to medical students or providing medical consulting to Siriraj Hospital. His interest is fully supported by his older brother, he said.