The teenage boy, who remains in hospital, was a passenger in one of two cars involved in the crash on the A61.
It happened on the A61 between Busby Stoop and Carlton Miniott at around 9.25pm yesterday.
It involved a green VW Bora and a black Ford Focus that was heading in the opposite direction towards Thirsk.
North Yorkshire Fire Service said crews from Thirsk and Ripon were dispatched at 9.30pm.
A spokesman said: “On arrival crews assisted with the release of a number casualties, who were trapped, from one of the vehicles. The causalities were then handed over to paramedics for transportation to hospital.
“We can confirm that the son of one of the firefighters who attended was involved in the incident, he is currently in a stable condition in hospital.”
Two 17-year-old boys, rear seat passengers in the VW Bora, were pronounced dead at the scene by paramedics.
They were earlier named locally as Mason Pearson and George Turner, but police are yet to confirm their identities.
Police said two other teenagers - one of whom is the firefighter's son - are in a critical condition in hospital, while the 22-year-old driver is receiving treatment injuries which are not life-threatening.
The driver of the Ford Focus has been discharged from hospital, but a female passenger is still being treated in hospital and two children are being kept in for observation.
In total, there were seven land ambulances, an air ambulance and coastguard rescue helicopter in attendance at the incident, along with police and fire crews.
Any witnesses are urged to email [email protected] or call 101, select option 2, and ask for the Major Collision Investigation Team.
Please quote reference number 12180039673 when providing details about this incident.