Check out FIRST Robotics. They are an international organization whose goal is to bring robotics to school children. They host a national competition every year for high school students that's really extensive (and expensive, the kits are $8,000+) but they have great programs for younger kids as well. I know that you home school so you might be interested in working some of the elementary school kits/projects into your curriculum later. I think they have a Junior FIRST Lego League program geared for kids as young as 6.
In any case, this would be a good resource for recommended kits. My daughter's robotics team ran a camp for elementary school kids over the summer that used Lego robotics kits that sell for $55. It might be a little advanced for someone as young as your son but could be something that you do with him. These kits are durable, flexible, and can be used again and again. The team was able to find lots of documentation on different projects to have the kids do and was able to accommodate kids of varying levels of interest and ability with the same set of base kits.
Hope that helps!