My guess is that most children who are brought to the US don't speak English. As long as you don't move into an entirely Spanish-speaking section of the city and put your kid into a school where most of the children speak Spanish, he or she should be fine.

I know that sounds backwards, but there are some areas of the country where children's exposure to English is so minimal that they don't learn quickly. Put him in kindergarten with other English-speaking kids. He'll soak English up like a sponge.