When Jacob was born eight weeks early most of his left arm was missing.
His parents Gemma Turner and Chris Scrimshaw, from Calderdale in West Yorkshire, crowdfunded to get a £16,000 functioning limb made for him.
The NHS and most companies take the view that a functioning prosthetic is not an option when the limb ends above the elbow.
That is where Ben Ryan, from Menai Bridge on Anglesey, came in, designing an arm for Jacob, who is now five.
Mr Ryan developed a hydraulic design after his son Sol had an emergency amputation when he was 10 days old.
It led him to quit his job as a psychology lecturer and set up his own company, named Ambionics, two and a half years ago.
His firm merged with Polish prosthetic maker Glaze this year.
One of their first clients was Jacob.
Mr Ryan has been working with a prosthetics expert and Jacob's family to perfect a hydraulic arm for him.
The family wanted an elbow that could be set in different positions, a gripping mechanism and a modular hand that can be swapped out for other tools.
He explained that the prosthetics are not 3D printed in the normal way, as they are forged together in a bath of nylon powder using lasers.
Mr Ryan said the elbow can be set using a sliding lock, and the hand closes when Jacob squeezes a water filled rubber chamber that is mounted to the upper arm.
He designed a mechanism to make it work while the arm was cast by his colleagues in Poland.
Perhaps, more importantly - for Jacob anyway - it is large, green and superhero themed.
"It was what Jacob wanted, including have a larger hand, so the theme is perfect," said Mr Ryan.
On Thursday he delivered the arm to Jacob at a meeting in Ringwood, Hampshire, and said the fitting was a "success" and that Jacob "exceeded everybody's expectations".
"He can give his brother a hug and hold his hand," he said.
Speaking after the final fitting, Gemma, a police officer, said watching her son wear the arm was "lovely", adding that he "really likes it, he's got it on right now".
She explained that Jacob did not want a non-functioning prosthetic and said: "He's not bothered about looking like everybody else."
The addition has also helped with balancing his posture, she added.
While raising funds to get Jacob a functioning prosthetic, one anonymous donor gave them £5,000 - saying she was terminally ill and unable to complete her bucket list.
Gemma said asking for money was "kind of a bit strange for us but you've got to do what you've got to do".
"The family have had so much bad luck getting help for Jacob," said Mr Ryan.
"Nobody has been able to deliver something that could work for him.
"It's always been the same status-quo - that it won't work when the prosthetic is for the upper arm."