Building Stuff- Build a Bot Kits?

If he wants to build home depot and lowes have free classes for kids with a different thing to build each month. You can also buy the kit and take it home. We have bought robot kits but my 10 and 9 year old needed help with those. You can google DIY robot and make it a family project.
I would also google beginner robot kit and see if there are any simplistic kits. Or check and see if any of those mail order toy companies have a robot kit-you can usually buy one kit instead of a subscription.

Our son's been a builder since he first put 2 pieces of duplo together.
Duplo is great for a few years, then he can move on to Lego - and Lego has quite a variety of building levels of difficulty.
It's great how they learn to follow the directions and they are free to do any free form building they want to do.
Erector sets are great too.
For our son, Grandma got him a set of Quadro when he was about 3.
We had to build it for him at first but soon he was building his own climbing structures (we always inspect for safety and stability before anyone started climbing).
There are lot's of building toys out there - you'll have to find what appeals to your son.
Try several different sorts and see where it goes.

We love Snap Circuits. My husband and daughter have the huge kit and she can put together quite a few projects now. It's not quite what you are looking for, but is a great intro to electrical engineering with fun projects.

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!

Please pay attention to the age requirments on the boxes. They are very complicated and will take an adult making it with them. That takes away from the child's experience. Go to google and find some science places that have kits for kids. We get dig and jewels encased in plaster and they use tools to get them out.