For what it's worth, I have spent small amounts of time in South and Central American schools over the past couple of years (although not Mexico and not in high schools). My impression is that state run schools are by in large strict, demanding, very structured, and in terms of content are often ahead of where my children were at the same grade level in their US schools. I can see any student coming from the US would have difficulty adapting. My belief: a student who was only 'doing good enough to play' academically in the US would have trouble even without the language barrier.
To me, what would be unfair is for him to have done well in the Mexican school and not be cleared to play here because of some bias against that school..