Good advice above.
If K is half-day in your LEA, you could also consider attendance at 5.5 in K for the half-day it is in session, and then 1st for the remainder of the day, as a transition to full 1st during that year.
I wouldn't worry about covering the whole 1st curriculum. I gather that you are talking about regular K entry in 2016 vs early entry in 2015. So he will definitely have mastered all of 1st by 2016 entry. There is negligible content (social studies, science) in 1st, and he is already well on his way to mastering the foundational academics of 1st.
