If you're calling your brother's US cell number from your US cell, you dial it like this:
001 + area code + phone number
That's the format for calling the US from Mexico, and since your brother has a US cell, that's what you do, even if he is IN Mexico when you make that call.
To call a Mexican number, it's 011 + 52 (country code) + area code + number.
P.S. I also have Verizon and make calls all the time, and this is how it works.