First of all, don't change the way you communicate with your wife. This wont have any effect on the child so it's unnecessary to mess with it. If you already speak both, or either, just do whatever works for you. Remember when you were young, even if it was in the same language, it was all "adult speak" and you probably tuned out anyway.
As far as the child is concerned, the general rule is that the better the child learns the second language, the easier it is to be flexible late on. Children usually do better if they associate each situation and/or person in their life with a language. For example; my kids speak to me in only English, only French with their father and outside (we're in France) and then German only at school or with German speakers.
What happens when you flip between languages is that the child finds out it's okay to use, usually the community language with the parent(s) and soon it becomes a battle to keep the minority language going at home. I didn't want this to be difficult so I keep to English in all situations, including in public, including in front of my in-laws, in front of my non-English speaking husband, etc.
I'm not rude about it. What is said to a toddler is kind of obvious. As they grew older, I taught them to be polite with English and only use it in small, usually obvious, exchanges. We never use French unless stuck on a word. My youngest is especially amusing with this. ("But I can't tell you because I don't know what it is in English!" "Well then what is it in French? Did you forget that Mommy speaks to Papa in French and has been in France 14 years??")
You have to decide whether you want to teach your son English at home, unnecessary if you live in the U.S., or whether you want to concentrate on the Spanish to really make sure he's fluent in both. Speaking a non-native language to your child is not always the easiest but that's a personal decision your wife can make for herself. You could start out all in Spanish and she could later switch to English once he starts school and starts getting exposure to English. In your case, he'll get plenty of English by just stepping out the door!
There's no need to switch to English outside the home. Not only would you miss a good opportunity to use vocabulary he doesn't have at home but then you get into the when-to-speak Spanish again dilemma. If you're 20ft from anyone else, do you continue in English? I saw a family at an airport in Germany, switching from Portuguese with each other, to heavily Portuguese accented British English with their child. Hmmmm... That was useful
You have to ask yourself, does everyone have to understand your private exchanges? Do you want to risk your son thinking that Spanish is something to hide or be embarrassed about? What will he think if he hears Spanish in public? Why are they using that and not us when we speak that way at home?
Not that one can avoid the occasional awkward moment. One time though, my son saw a Hasidic Jewish man giving a speech in the U.S. My son is used to seeing Hasidim in public in France but not in the U.S. (especially not in California!) He tugged on my arm and in a loud voice said "Mommy look! He speaks ENGLISH!" So we had an American child telling his American mom that a fellow American spoke English, in amazement while in the U.S. Yes, sweetheart. There are Jewish people in American too, like... your own mother?
I do read aloud only in English. When they were little, I would just tell the story in English but later I taught them which books were which. "Now ask Papa to read this to you because it's in French". Again, it was the littlest who protested. "No! It's in English!" even though she couldn't read.
I do try to get my kids to watch videos in English. Our "rule" is to watch it in the original language. I'll let the little one watch "Harry Potter" in French because it's kind of hard for her (special vocabulary, etc.) For fun though, I noticed the other two go back and forth, just curious how it's translated.
But I do let them watch cartoons in French, or for that matter, whatever language. I've been on vacation and found them watching cartoons in the local language. At my parents, they'll watch them in Spanish, just for fun.
They speak to each other in French and I wont stop them. There is a mom at their school who makes her sons speak to each other only in German but I never did that. Part of the reason is that we're near Germany and her dh is fluent. In fact, I think they speak to each other in German so it's easier to have one "family language". We don't. Our dinner table is a mismatch but we're used to it.
It's not too early to look around your community and see what is out there to help this project. Spanish speaking schools, playgroups, organizations, babysitters, etc. can help show your son that Spanish isn't just what his parents use.
These schools can have waiting lists and in bilingual programs, they usually favor native Spanish speaking children for places. Do take time to look into this matter. Some "bilingual" schools might be to just teach English speaking children Spanish and could be boring for a child whose fluent. "Dual Immersion" is best, if you can find it. Most young families do move at some point so knowing about the schools could be a factor in where you next live, or whether to stay in your area.
The secret is to do what works for you. Whatever you can keep up long term should be the goal (although, as I mentioned, the rules can be changed if necessary). Living in the U.S. with a native English speaking mother really favors the English in your son's life so whatever you decide, it might be helpful to keep in mind that the Spanish might need more support.
Some people will say to just do whatever but honestly, in RL, I've seen families flip back and forth and then the children switch to the community language, soon losing the other one quickly. I really think that being disciplined really helped to insure that my children were truly bilingual. It's never been a battle and it just feels normal to us.
It's so nice to go back home and have my children run up to my parents speaking as if they never set foot outside the U.S. (let alone live and were born in France). My son recently passed a native-level test for an English program for middle school. He turned it down to continue in French-German bilingual school. He reads age-level books in English although he has never been schooled in English. It's not just a language you're teaching your son but a whole culture and outlook on life.