Police have been told the 20-year-old man was in the tray of a white Holden Colorado utility, with another young man, on Western Tyers River Road when the ute hit a large bump around 7pm.
The Clyde North man was ejected from the ute tray onto the ground and was seriously injured.
The driver and other man in the utility were able to raise the alarm and drive the injured boy to Tanjil River Road to seek medical care.
The young man was airlifted to a Melbourne hospital in a critical condition.
The 20-year-old driver from Langwarrin was arrested and remains in custody this morning.
Investigators are appealing for anyone who witnessed the group driving in the area throughout the day to come forward to police.