L.T.
Frankly, the experience of living and going to school in another country seems to be too profound (and educational)an opportunity for a young child not too miss. The challenge of going to school in another language will probably make the experience pretty challenging even if the material is not.
Find out what the math and reading curriculum would be if he were in 3rd grade in the US and use that to gauge if he's meeting the benchmarks in the Mexican school. If not, go ahead and supplement the curriculum yourself by making sure he's reading books at or above his grade level. Talk with him about what he's reading. Take some age-appropriate math games so you know he's getting the fuctions that he'll need to know.
The WASL isn't required for graduation until high school. A lot of schools "teach to the test" so once you came back to the US he would probably be well prepared to take the WASL.
Personally, I wouldn't dream of passing up the cultural experience of living in another country. Not because of a standardized test anyway. Your children are very fortunate.
Good luck with your decision (and good luck on working things out with their dad!)