When Ryusuke Sekino was born last October, his mother feared that even touching him could prove dangerous for his tiny frame.

Six months later, Ryusuke, believed to be the world’s smallest surviving baby boy, is preparing to leave hospital in central Japan on Saturday after his weight increased from 258g (9.1oz) at birth – roughly the weight of a pack of butter – to more than 3kg.

He was born two months after the birth, also in Japan, of the previous record holder, who weighed 268g. The infant, who has not been named, was discharged from a Tokyo hospital in February.

Ryusuke was delivered on 1 October by emergency caesarean section at 24 weeks and five days as his mother, Toshiko Sekino, was suffering from hypertension.

The newborn, who was just 22cm in length, was treated at a neonatal intensive care unit at Nagano children’s hospital, where he was fed via tubes and with occasional drops of his mother’s milk administered with cotton buds.

“When he was born, he was so small, and it seemed as if he would break if I touched him. I was so worried,” Toshiko told reporters at the hospital on Friday. “Now he drinks milk. We can give him a bath. I’m happy I can see him growing day by day thanks to the special care he received here.”

Takehiko Hiroma, a doctor at the hospital, said he was “very happy” that Ryusuke’s treatment had been successful. “There have been difficulties in the treatment because, immediately after his birth, his blood vessels were too thin to administer intravenous drips,” Hiroma said, according to Kyodo news agency.

The smallest baby to have survived was a German girl who weighed 252 grams at her birth in 2015, according to the University of Iowa, which keeps a registry of the world’s tiniest surviving babies.

The survival rate among very small babies is significantly lower for boys than for girls. The university’s website shows that all but six of 25 babies worldwide who survived after being born weighing less than 300g were girls.

Agence France-Presse contributed to this report.