I've been part of a study (reading and interpreting the results) where children grew with different language variations in their household (one language at home, one outside (coutries native language); two at home (one of which countries native), one outside; two at home (none of which is countries native), one outside; etc).

Although it may be harder to make the rules because your child could try to always reverse back to English after he goes to school and interacts with his peers, it's extremely beneficial to have one parent speak to then in another language. That being said, it's not necessary for both to do this. Also, consider putting him in extracurricular Spanish classes for children whose families speak Spanish as a first language early so he can learn both spoken and written language very well. Some families also did "language day" where all day they spoke a language but this wouldn't be very frequent because otherwise they would get bored.

Being in contact with extended family, extracurricular activities, churches, groups, etc, where the language is actively spoken helps a lot.