I am not qualified to give you a real response as I am not a professional in this field in anyway, but I am bilingual and my mother taught me how she made both my sister and I fluent in both Korean and English.
First of all, she made sure only to speak in Korean with me, while I was learning English in an American school.
She also made sure that I NEVER mixed both languages and had me speak either fully in Korean or English. This was extremely important because it made me learn words for both languages rather than cherry picking easy words.
She taught us elementary school level Korean during summer and winter vacation while we were in elementary school (I only went to an American school).
Finally she exposed us to Korean media by leaving the TV on Korean shows while we were playing. We would constanly be exposed to Korean at anytime, and would tell us what certain words that was spoken on the TV meant.
Based on my experience, I would suggest speaking to him in the language he wont be learning in kindergarden (Spanish) as he will naturally speak English with his friends and teachers. I would also suggest to keep an eye on his educational progress in English and teach at a similar level education in Spanish.
Make sure he NEVER mixes both languages when he speaks and make him choose to either speak in English or Spanish. You could also watch some videos together in Spanish and teach him words that he does not know the meaning of.
It seems you are planning on having your son be educated fully in English through the school system, which would mean he would communicate in English most of the time outside of your household. Which would make English his dominate language (As I am).
Again, I am not a professional in this area in any way and this is 100% based on my personal experience.