It really depends upon why he is stopped.
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When you say tourist visa are you also saying that he has a valid license to drive a vehicle? Without a license he can't drive, period! Assuming he has a license , he would get the ticket and they you would hire an attorney to deal with the infraction ticket, speeding, failure to obey signs etc. If he got a more serious crime then you would hire an attorney in your jursidiction to fight that ticket. This is what you would do.
If your brother is over 16 years, 3 months old, and he has IN HIS POSSESSION a valid driver's license issued to him from Mexico, he may legally drive in Virginia. See Va. Code section 46.2-307. If he does not have a valid driver's license, then under certain circumstances you can be charged with a crime if you knowingly let him drive your car.
If he's got a valid license with him, it would be a good idea for him to keep his passport with him when he drives.
Some traffic stops end in arrest regardless of citizenship, such as when a DUI charge is brought. Most likely, if it is a simple speeding ticket, your brother would receive a summons for court and be released on the scene.
If he is charged with something specific, you both should contact an attorney to determine what your next step should be.