Where I live declaring your kids as homeschoolers just means that the government keeps track of information about the age of your child and the fact that they're not in school. It doesn't say anything about their academic level. If your child is listed as "1st grade" that merely means that based on age that's where your child would be placed if in school. When homeschooled kids enter the school system they're put with their age-mates. If that placement turns out to be inappropriate one would assume that they would consider adjusting the placement if that would be in the best interest of the child, just as they would with a gifted or learning disabled or ESL child, or one moving from a country with a totally different educational system.
My homeschooled 8-year-old is doing 6th grade work, but is considered a 2nd-grader by the officials in our school district. As I see it that's great: it gives us the wiggle room to adapt the pace and direction of her education with absolutely no fear that she would fail to measure up to grade level if assessed. In two years she'll have to do standardized testing as a 4th grader and I know it will be a cakewalk for her with no need for preparation or stress. That's exactly how I want the government's silly standardized testing to be ... easy and not intruding in the slightest upon her homeschooling.
By all means, start "homeschooling kindergarten" with your little guy in the fall if you want. But I would not legally declare him as a homeschooler until the following year (or whenever your state requires it). No point in subjecting yourselves to more regulations and scrutiny than you are legally obliged to accommodate.