Wee Oscar, as the five-year-old is now affectionately known by his legions of Twitter followers, has been battling an aggressive form of cancer called neuroblastoma since he was just three.
In just over a year, the little lad has had 30 blood transfusions and almost 140 platelet transfusions.
While his family say he simply would not be here without them, they are all too aware of the shortfall in meeting demands for blood.
While blood transfusions can be required for mums in childbirth, newborns, people undergoing surgery and those like Oscar battling cancer, just 6% of people in Northern Ireland actually donate.
But on Tuesday, the Great Hall was packed out with people keen to make a difference.
Belfast's Sinn Féin Lord Mayor Máirtín Ó Muilleoir and DUP Deputy Lord Mayor Christopher Stalford led by example in joining those who were giving blood for the very first time.
"The outpouring of support is amazing," Mr Ó Muilleoir told UTV.
"All day, people have been turning up to give blood and to back Wee Oscar."
Oscar's parents, Leona and Stephen Knox, were delighted with the turn-out at the City Hall blood drive, having addressed the council back in November when the plans were first proposed.
"We hoped for this response, but to actually see all those people ... It's just incredible," Leona said.
Unfortunately, the wee man himself was unable to be there as he recovered from a chemotherapy session at the weekend - but his dad is sure he will be equally delighted.
"We'll show him it on the news and tell him that was all for him," Stephen said.
"The main goal from today was to try and get as many new donors signed up as possible, who will then hopefully become lifetime donors."
Motorcycle racer Michael Rutter was among the familiar faces keen to get involved, having heard about Wee Oscar through a friend.
"Hopefully it helps," he told UTV, already hooked up to give his own blood donation.
"And not just Wee Oscar, but lots of other people as well. I've got two children of my own and it just brings it home."
Belfast Giants captain Adam Keefe was also on hand to help spread the message, with Wee Oscar having become a popular visitor to the Odyssey to visit the team.
"I can honestly say I'm a better person for having met Wee Oscar - he's an inspiration to everyone," he said, recalling how the little boy had brightened the team's day during a visit to the Odyssey.
"I think a lot of people don't understand how easy it is, or what the process is, or if you're even able to give blood.
"It's really important to just get down - if not here, then to a clinic - and find out if you can give."
People like those who have turned out today have saved Oscar’s life. Parents like us rely on strangers to save our children’s lives.
Leona & Stephen Knox
The Northern Ireland Blood Transfusion Service is hoping that the boost to donor numbers will not be a one-off.
They know people may have experienced queues during Tuesday's session, given the exceptional circumstances and are keen to promote the many other opportunities there are to give blood.
But, according to NIBTS spokesman Paul McIlkearney, one thing is certain: "Oscar has encouraged people to give blood - Oscar has actually saved a lot of lives himself."
Generally, it takes just 10 minutes to collect the amount of blood given in a session - which is just under a pint - but donors should allow up to 45mins for the whole visit.
While certain medical requirements must be met, you can give blood if you are over the age of 17, and under the age of 66 if you are a first-time donor.
Within days of giving blood, you could help to save a life.
Information on the requirements can be found on the NI Blood Transfusion Service website, where details of other sessions available around Northern Ireland can also be found.