Also, be sure to talk with your child's teachers about how to enhance both languages at home and school, without causing any kind of language confusion. There are many great resources in schools to help you. (Ask for ESL counselors in your district).
How old is your child? That makes all the difference. My son is 4 now and I always spoke to him in spanish, so he understands, but since everyone else speaks to him in english that is what he speaks. Speak to your son in both languages. Say the name for things in both English and Spanish. If he's 2 or younger he'll pick both up quickly. Once he starts preschool, he will be more inclined to speak English if that is what most kids are speaking, KEEP speaking spanish at home! I always though, I still have time and now here we are 4 years later and my son isn't as fluent as I would have liked. :-/
Good luck to you! I found that listening to music in spanish and watching his favorite shows translated help too.
I also bought this book, it's fun and it gives him a good visual.
Does your son attend an all-English speaking school? If yes, then I would definitely recommend you to continue speaking Spanish at home. Please don't replace Spanish with English at home. He needs a solid base in Spanish so that he can learn English. Without the base, he will struggle with both languages in the future. This isn't to say don't speak English at home. It will be beneficial to read books in both English and Spanish or you could pick a time of the day or a topic that should be spoken in English. For example, when talking about school, you two could speak in English but for other topics, you can continue to speak in Spanish. Or the conversation while waiting for the school bus can be in English.
I grew up in a non-English home environment but my education was all in English. I'm currently now fluent in both languages. My parents didn't have to send me to additional classes after-school or on the weekends, and I am thankful for that. I learned my native language from my family and English from my teachers and friends.