He had bronchiolitis and a chest infection, but with the added complication of a tiny hole in his heart.
A year on, the bright little boy from Knaresborough is unrecognisable as the same frail newborn who spent two weeks in a critical condition.
And his family, determined to give thanks, have hailed the NHS at its best as a miracle-worker.
"They saved his life," his mother Rachel McNab, at the close of a mammoth fundraising challenge to give something back.
"Twenty years ago, we would probably have lost him, it was that serious.
"Everyone in that hospital, from the top consultants to the support workers and catering staff and cleaners, they are all cogs in the machine," she adds.
"They are all vital to making a hospital work. And they've been fantastic. I'm not a dramatic person, but they did save his life."
Finlay, born in late November 2018, had developed bronchiolitis at just a few weeks old and, when his mother took him to the GP, his heart murmur was picked up immediately.
But on December 6, he was admitted to Harrogate District Hospital with a bacterial chest infection and put on a special machine to help him breath.
"Because his heart was struggling, and a small hole in a tiny heart is actually a big hole, all of his energy was going into his breathing," said Mrs McNab, 35.
"I think that was the hardest bit, there was no change in him. He was on the critical list. They couldn't transfer him to Leeds because they only had oxygen, even in the helicopter, and they didn't think he would survive the journey.
"You don't realise how bad it is at the time, you just focus on the next day and the next."
In the days that were to follow the McNab family, father Aidan, 37, Charlie, 13, and Freya, three, were in a state of constant motion as they tried to ensure there was always someone by Finlay's side.
On Christmas Eve, Finlay began to improve. By Christmas Day, he started to feed again, and he was able to go home the day before New Year's, to a cheer from nursing staff.
"The hospital was amazing, from the very beginning," says Mrs McNab. "I was a hormonal mess arriving, having just had a baby.
"They were so warm and welcoming. They couldn't do enough, for anyone. They make such a fuss of every child, they make every child feel as if it's a nice place to be.
"But they are completely under-resourced for the area they cover."
The extended McNab and Finan family, spurred on by Mrs McNab's sister Hannah as its 'driving force', pledged a year of fundraising to support the hospital's charitable trust.
Having set a goal of £5,000, all ambitions were smashed with the support of an entire community in Knaresborough which has backed their appeal.
Together they have raised £8,500 for a cardiac monitor for Woodlands Children’s Ward.
There were donations from local businesses such as Heck, for a summer barbecue, window cleaner Nick Brown did a coast to coast cycle ride, and Halifax bank matched all donations it raised through cake sales and book drives.
A dance academy in the town hosted children's parties and raffles, there were sponsored races, coffee mornings, half-marathons, a sky-dive and the Yorkshire Three Peaks - twice.
"It's incredible, the kindness of people," said Mrs McNab, overwhelmed by the generosity of an entire town which has given what it can as thanks for Finlay's care.
"It is a small community in Knaresborough, and we've all grown up here, but so many people and businesses have helped to support us."
Finlay, now 14 months, is rapidly improving as he grows and his visits to hospital, though still intermittent, are getting further apart and shorter.
"He's a little trooper," says Mrs McNab. "He's into everything, climbing, refusing to walk because he can crawl faster to get where he shouldn't be.
"The hospital was amazing and they did save his life - every single person on that ward.
"It's not just Finlay's life, it must be thousands of children whose lives they've saved over the years.
"If we can do something to help them with their jobs, that's the least we can do."