Abby Roby describes seeing her son in a coma as a "parent’s worst nightmare."

Tristan Roby was struck by a vehicle in a hit-and-run while riding his bike on Exeter Road on July 21.

The 18-year-old Laurier Secondary School graduate has been unresponsive to pain since the collision.

“He's stable enough to put in dye to see how much blood flow is going to his brain,” his mother says. “If there is not enough, we’re going to have a bad conversation after.”

She adds that if the flow is moderate they can use medication to help, and if it is adequate they will drop him off sedation.

And if that is the case, “Hopefully he’ll wake up in the next few days,” she says.

While Tristan remains in hospital, the London Police Service Traffic Management Unit is handling the case.

Sgt. Sean Harding told CTV News last week that they were following up on leads, but the driver of the vehicle has not been arrested.

While one passenger was taken into custody at the scene and the vehicle owner has been questioned, Abby says they still don't know who was driving that night.

Abby is asking that anyone who has information come forward, saying “I think my son has the right to know who drastically changed his life.”

And when Tristan does wake up, his life will be different.

“He literally has a ping pong-size hole of tissue in his head, so we don't know what effects that is going to do. They are saying it’s somewhere along the line of Cerebral Palsy. “

Abby adds that it’s difficult to see him hooked up to the tubes, and not breathing on his own.

She says knowing he’s sleeping can be a blessing and a curse at the same time.

“Your brain wants you to feel better, but eyes are telling you it’s a dire situation.”