Lee, I think that is awesome that you found such a program and are giving your son the opportunity to continue learning (and using) his birth language. Like most bilingual children, he obviously associates Spanish with school and English with home; most children associate a language with inside the home and outside the home or one language with one parent and another language with another parent. In an immersion program, the immersion language is used at all times and in every environment, whereas the second or third (English) language is used only in designated rooms with designated teachers. The children are never confused by this. Only adults perceive confusion with our adult minds.
I would be willing to bet that your son knows a lot of Spanish; he just does not use it at home, because he thinks it is not the right place. Take care!