Options? OPTIONS! Options will drive you CRAZY! We have SO many options to choose from and learning what works "best" for you and your child usually takes a lot of trial and error. Most of us start out trying to "do school" at home.
You have been teaching your son for 5 years already!
My advice would be to ease into it. Those K12 programs are quite intense and require a lot of time. The basics of what kids learn in different grade levels can be found by searching "scope and sequence" for that grade. Here's one for K.
http://www.hmhco.com/~/media/sites/home ... .pdf?la=enEvery publisher of curriculum should have one. They are guidelines you can use. Or check out a book like "What Your Kindergartener Needs to Know" from the library.
Just please do NOT spend a lot of money on curriculum at this time. He's still a kid and there's lots of time to learn to sit and do school. He should be active and explore things he enjoys. If you live in an interesting area be sure to plan "field trips" to learn about your area. Your address and phone number, parents "real" names, .... beginning life skills are just as important (if not more so) than academics. Have fun!