What is a good programming language to start my Grade 1 son learning?

I'm probably the only one here with this opinion, but I think 6 is too young to start a child on programming. Those years are critical for development of a whole host of skills including social skills that are not computer-related (that, indeed, may be antithetical to computer use) and intellectual ones that actually will contributed to computing skills later on (I'm talking about math and problem solving skills).

I've started introducing my kids to programming at the ages of 8 and 10, but I don't expect them to take a serious interest in it until their middle school years (starting at age 11/12). In general my kids spend much, much less time in front of a computer than their classmates. They both lead their classes academically and are well socially adjusted.

Take a look at Small Basic from Microsoft.

By providing a small and easy to learn programming language in a friendly and inviting development environment, Small Basic makes programming a breeze. Ideal for kids and adults alike, Small Basic helps beginners take the first step into the wonderful world of programming.

I think the quote sums it up, really! :)

Try Scratch.