Obviously, with the swine flu, that's not a good idea. But, it could be gone by July, so you might want to take a "wait and see" attitude.
Mexico is a great country to visit, whether you're visiting the beach or the mainland, touristy or rural. Lots of great culture, traditions, food, etc. However, I don't see why a 5-year old would enjoy going there. Has he mentioned anything about where he plans to go or what they plan on doing in Mexico? That would be my biggest question.
My next point - is he originally from Mexico? You should know that in Mexico, it is very common (practically acceptable) for fathers to kidnap their children from their mothers and the law is practically on their side. I'd hate for your son's dad to have very bad intentions with taking your child to a country where they can "disappear".
Also - children cannot cross international borders without notarized consent from any parent that is not traveling with them. So, your son's father cannot legally take him across the border without your permission.