He believes there is between an 11 and 56 percent chance the nucleoside therapy would improve the baby's muscular strength and that there is a "small but significant" chance it would also help brain functions.
Rare-disease specialists at Bambino Gesu' are working with other global experts to map out an experimental treatment protocol for Charlie, hospital chief Mariella Enoc said.
Charlie's parents Chris Gard and Connie Yates also attended a meeting with Hirano and the GOSH medics, according to The Sun.
The couple want him to undergo a therapy trial overseen by Dr Hirano in NY, which GOSH specialists don't think will help.
Great Ormond Street Hospital requested a new hearing after the New York-based neurologist claimed his treatment could now have a better chance of working.
Charlie's parents are fighting to take him to the U.S. to undergo nucleoside bypass therapy, believing it could treat their son's extremely rare mitochondrial DNA depletion syndrome.
"Despite all the advances in medical science made by GOSH and the other hospitals around the world, there remain some conditions that we can not cure and we can not ameliorate".
A sketch showing Connie Yates and Chris Gard listening as Professor Hirano offers experimental treatment for little Charlie
The case has gained global attention after interventions by US President Donald Trump and Pope Francis, who have both voiced support for Charlie.
In light of Dr. Hirano's visit, a source close to Charlie's family said they "remain optimistic" about treatment.
The couple, who are in their 30s and come from Bedfont, west London, are mounting the latest stage of their fight at a hearing in the Family Division of the High Court.
In addition to evaluating Charlie, Hirano will meet with doctors and others, on Monday and Tuesday, who have been caring for him.
Today Dr Michio Hirano, an American neurosurgeon, who claims to have pioneered an experimental treatment he says can help Charlie, will examine the boy after flying in from NY.
The examinations of the next 2 days will likely provide the information needed for the judge to make a final decision on the case.
While he has not seen the baby in person, he said that the tests on Charlie's brain show "disorganisation of brain activity and not major structural brain damage".