So what your saying is I have no chance?! THIS IS SOOOO UNFAIR I DON'T WANT MY SON IN A FOREIGN COUNTRY WITHOUT ME. What if they don't come back!!! grrr
I wouldn't say that you have no chance. It comes down to the particular judge and whether your reasons for opposing the trip are valid.
If, for example, there is risk of flight, Mexico is not a good place for a parent to take a child. Mexico does not cooperate very well with the Hague convention:
International Divorce
The U.S. Department of State has a travel advisory about travel to Mexico. It would depend on where they're going and how they're getting there.
If Dad wants to take the child, he's going to have to file for a court order to get permission and ask the court to order you to sign the appropriate documents. You will have the chance to present arguments as to why it shouldn't be ordered - hopefully including reasonable arguments like the above rather than simply throwing a tantrum. It will then be up to the judge to decide.
Is there anything in your existing custody order about travel out of state and/or out of the country?