Hi Zia-
I'm glad to hear you ask about "developing a curriculum" and not buying one. Single grade level "canned" curricula rarely are a good fit for gifted kids, especially at the higher levels of giftedness. Most HG+ kids are at least somewhat asynchronous, and do better learning with a variety of materials cobbled together.
The first thing you need to do is to educate yourself about the law regarding homeschooling in your state. If you don't have to register him until age six or seven, then you can do anything you like. If he needs to follow state guidelines this year, then you have to ensure you've got the basics covered.
It sounds as if you are already doing a fantastic job of providing stimulating books, computer games, etc. You might not have to change anything but just keep on facilitating this way.
If you want a specific program for history, you might consider Story of the World. It presents history in a logical, chronological manner, with a chapter of reading followed by corresponding questions and activities via the companion activity guide. My kids use SOTW as a spine and we do lots of supplemental projects and reading.
For math, I recommend EPGY through Stanford if money is no object, or Singapore workbooks if you have a budget. Both programs are excellent and you can find many positive reviews on them. See
www.singaporemath.com for sample pages and printable placement tests.
Sorry, have to run, but let me know of this is helpful and I may be able to add some ideas.
