Unanderra's Zachary Matheson weighed a tiny 960 grams at birth, so when his mother's cat had kittens he begged and pleaded to keep the runt of the litter.
Over the last three years Zachary, who has autism, has formed a strong bond with "Junior" and is devastated after the cat went missing two weeks ago.
His mother Carlie urged anyone with information about the black and white cat - last seen in Albert Street - to come forward.
"Zachary was born 14 weeks early and was so small and so when my cat had a litter he immediately zoned in on the tiniest one and begged to keep him," she said.
"Zach has been diagnosed with high functioning autism, attention deficit hyperactivity disorder, global developmental delay and sensory issues.
"He can't handle bright lights or too much noise. But what he loves to do is sit quietly with Junior and watch television or play on his iPod.
"If he gets upset or stressed out, Junior will come and sit next to him and just seems to calm him down. He's his 'pets as therapy' cat."
Ms Matheson has put out a call on social media, including the Illawarra lost and found pets Facebook site, in an effort to bring Junior home.
"It's been a couple of weeks' worth of anxiety and high emotions," she said. "Zach's really fretting and he's not eating or sleeping well. We really want him back."
Zach, an Aspect South Coast School student, said: "Junior means a lot to me. We play together and he sleeps in my bed. He's like my little brother. I hope he comes back soon."