Police have arrested a 28-year-old man for attempted murder after a five-year-old boy went missing Thursday morning from his Brampton, Ont., home.

Peel Regional Police say Dyon Smart, also of Brampton, is known to the boy and his mother. He faces charges including aggravated assault and breach of probation.

Smart appeared in court Thursday for a bail hearing.

Const. Sarah Patten said the "huge" investigation will involve a lot of resources.

"We are going to get to the bottom of this, hopefully as fast as possible. We are glad we were able to make the arrest today because of how big of a deal this is. This is completely unacceptable that this happened to a five-year-old child," Patten said at a news conference on Thursday afternoon.

We are going to get to the bottom of this. — Const . Sarah Patten

Around 6 a.m. ET, the boy's mother called police after she woke up to find the front door open and the child gone. Police were told the child was last seen at approximately 2 a.m.

There were no signs of forced entry into the house, according to police.

A police canine unit found him near train tracks close to his home, in the area of McHardy Crescent and McMurchy Avenue South, just before 7 a.m.

Police say the boy was found unconscious with life-threatening injuries.

It just doesn't make any sense. — Kevin Joseph, neighbour

Paramedics took the boy to Brampton Civic Hospital. He was then transferred by Ornge air ambulance to Toronto's Hospital for Sick Children in critical condition, according to paramedics spokesperson Brad Bowie.

Police say he is now in stable condition.

"In terms of how and where exactly this all took place, that's still part of our investigation and we're not speaking to that just yet," said Const. Harinder Sohi.

He said Smart was arrested just after 8 a.m.

Patten said the force's special victims unit is involved in the investigation.

'He was just… limp'

Neighbour Kevin Joseph was in bed when the five-year-old's mother ran over to Joseph's house around 6 a.m., "banging and screaming on the door" that her son wasn't in his bed, Joseph told CBC Toronto.

He joined the search in the community, and of the adjacent railway tracks, with police.

"I started looking with them in the bush, and then all of the sudden the drone was in the air spotting something and the dogs ran towards it," he said.

"And there was, like, nothing there when we found him. He was just, like, limp."

The boy is a "good kid" who was "always at his mom's side," Joseph said.

Police set up a perimeter around the area early Thursday. (Paul Smith/CBC)

He said his young daughter often plays with the five-year old.

"It just doesn't make any sense to me."

A perimeter has been set up around the area for an investigation.

Police are canvassing the neighbourhood for "every possible video feed" or any information, Marttini explained.

The railway has been shut down temporarily as a precaution.