I think it's about 90% certain that his grandparents were illegal unless they grew up in San Antonio. That's the problem with the BP, which is heavily Mexican-American, as well. Their relatives were/are illegals, and they identify with those sneaking into the US.
That's quite a reach. My best friend's grandparents came across the border as fully paid, ticketed passengers on a train in the early 1900s. How that constitutes "illegal", I don't know. Further, American born hispanics of Mexican ancestry don't as a rule identify with illegals any more than German Americans in fighting in WWII identified with Nazis. Maybe you should look at this: Hispanic American Group Joins House Leaders to React to Pres. Bush's Address & Oppose Senate Amnesty