he is American too, technically
He is a United States Citizen by birth. There’s no technically there. If he doesn’t have a US passport, get him one soon. It will make a big difference later in his life should he need it or want it.
Back to your op question. Which school district would be best is difficult to say. But, perhaps a place with a large Mexican population in a large school district not too far from Mexico would make sense. Look into California’s Coachella Valley area. The main cities there are Palm Springs, Cathedral City, Rancho Mirage, Palm Desert and a few more. In the Coachella Valley there is a population of over a million people, but it has a certain “small town feel to it”.
Many Americans use the term “Hispanic” as if it’s all homogeneous. While they do share a common language, there is a difference between Cubans, South Americans and Mexicans in culture. I’d assume you’d want a more Mexican culture and that area has a Mexican influence. It does get a tad bit warm in the summer (so much so that they won’t even admit it’s hot if it’s under 100). But, it doesn’t rain much.