The Toyota Rav 4 car he was travelling in collided with a lorry on the A4146 Stoke Hammond bypass at 14:17 GMT on Friday, police said.
Fire crews and air ambulance attended. The road was shut for several hours.
A 33-year-old man from Milton Keynes was arrested on suspicion of causing death by dangerous driving and released on bail.
Three people were taken to the John Radcliffe Hospital in Oxford where they were treated for serious injuries.
Police have appealed for witnesses.
A seven-year-old boy has died in a car crash on a dual carriageway in Buckinghamshire.