Without a doubt, study after study has shown the powerful advantage of wearing a seatbelt. Not only are there fewer deaths, there are also fewer and less severe injuries.
Your brother is correct that the shoulder harness can cause neck sprains during a motor vehicle collision (MVC). In fact, the number of people who develop neck sprain after MVCs is much higher than in groups who do not wear their seatbelt.
The downside of this statistic is that spinal fracture is much higher in the unrestrained group. So although they are less likely to sprain their necks, they are more likely to become a spinal cord injured patient.
Young men between the ages of 18 and 30 are the most likely to engage in risky behaviors. And driving or riding in a moving vehicle without a seatbelt is considered risk-taking. They are also more likely to drive at higher speeds resulting in more serious accidents.
The bottom-line is that the seatbelt may cause a neck sprain. However, it's more likely to save lifes and protect people from becoming a spinal-cord patient.