The family has set up a Givealittle page to raise funds for specialist equipment to improve his co-ordination and help him participate in regular childhood activities, such as playing on the swings and picking up a rattle.
Once Cassius has outgrown his equipment, Courtney plans to donate it to the community toy libraries to be used by other families.
"Cassius has beaten the odds so far, and with the right equipment, he can keep beating the odds," Courtney said.
"Charlie and I just want to know we did everything we could to give him the best life possible. Isn't that any parent's goal?
"We don't want pity; we just want to spread a bit of hope, and give back to the community."
Cassius was born on November 24, 2014, "a beautiful bundle of rolls" with luscious hair and liquid brown eyes.
At six months, he began having seizures "out of nowhere" and was in and out of hospital as doctors battled to find a diagnosis.
A brain scan revealed Lissencephaly, which came as a "huge shock", Courtney said.
"We were told he wouldn't be able to walk or talk, or show emotion, and he wouldn't live past his second birthday.
"We were sent home and told to make him comfortable."
The next few months were a whirlwind, as his family struggled to adjust.
Cassius was placed on several different medications to control his seizures, some which left him in a vegetative state.
Respiratory issues meant he had to be placed on a monitor every night, and severe food allergies have required emergency supplies of adrenaline and frequent trips to Carterton Medical Centre.
"It was like a tornado combined with a hurricane," Courtney said.
"He needed the exact dose of medication at the exact time.
"If he has a seizure, his rescue medications can stop him breathing.
"You get so used to having structure - but we've had to abandon that now."
However, Courtney and Charlie's spirits were lifted when Cassius began achieving small milestones.
He was able to roll over, open his hands to feel the textures around him, lift his head, and communicate his emotions.
His movements have further improved thanks to hydrotherapy, which Courtney has continued at home after her parents bought them a pool.
"He kicks his legs in the pool, and wriggles his arms around.
"He smiles and laughs, he smacks his lips, he put his hand out to feel the carpet, and if he's hungry or grumpy he lets us know."
Cassius also enjoys his food and can, unlike many children with the condition, eat without a feeding tube.
"Whenever we feed him yoghurt or custard, he makes sure every last drop is gone."
Plus, much to his parents' relief, he now takes only one medication for his seizures.
Courtney said one of the biggest difficulties is transporting Cassius, as he cannot sit unsupported and has outgrown his wheelchair.
With the Givealittle funds, Courtney and Charlie first hope to buy a Go-To Seat, which can be inserted onto shopping carts, swings and dining chairs, and allows the child to stay upright comfortably.
Later, they plan to buy an UpSee, a sling which allows the child to stand and walk with an adult's help, a high-backed swing seat, and a Firefly Scoot, which lets a child push himself forward while on his stomach.
They also plan to investigate silicone bands which will help Cassius pick up and hold his toys.
"And a lot of this equipment isn't available in New Zealand, and it's expensive," Courtney said.
"But we want to see him keep progressing, and to give him the opportunity to do the things other kids can."
While life has been exhausting, Courtney said she wouldn't be without her "happy little man".
"He has enriched our lives," she said.
"I say that if he can wake up smiling every morning, then what's my excuse?"
To donate to Cassius' family, to go to www.givealittle.co.nz/cause/mastercassius
