Wow, this is the perfect question for me to answer. I taught programming at a computer camp for several summers, from the end of high school through college. Many of the kids I taught were the same age as your son. And moreover, unlike some computer camps which have have more of a classroom computer-science curriculum, the one I was at was based on self-motivation and kids choosing projects that appealed to them. So in other words, I spent a lot of time teaching kids how to make games.
I would highly reccomend the program Microworlds, which is a version of the programming language Logo. Unlike many of the other suggestions listed so far in this thread, Logo was a programming language specifically designed for teaching programming to kids. Also, it makes it really easy to do basic graphics, which are much harder to do in most other languages, and which are of course essential for most games. I’d say Microworlds is the best possible place for a kid your son’s age to start learning programming. (Although I’m a little biased, because not only did I teach it for several years, but that – actually the older version Logowriter – was how I first learned programming myself at the age of 11. But I guess that shows it works – from there I was able to move on to such languages as C++, Visual Basic, etc.) Microworlds is probably the sort of thing where you can sit down and learn it with your son fairly quickly-- not being a programmer yourself, that may not be true of many other languages. Also, programming can be an inherently frustrating experience at times (since fixing errors in a program often takes a lot longer than writing the program) and especially for a kid with Aspergers, I would think that you’d want to minimize the level of frustration – which is why I think that using a kid-oriented language like Microworlds would be the way to go.
I’d also recommend that you get in touch with my former boss, Karen Rosenbaum, who’s been running a computer camp since 1983 where many, many kids your sons age have learned to program. I’m sure she’ll have lots of good advice, plus she hires college students from all over the U.S. (and even a few from the UK, Ireland, and elsewhere), so she might even know someone in your area who might be willing to tutor your son. You can get her contact info off the website for her camp, www.ticcamp.com If you happen to live in the D.C. area, maybe your son would even want to attend the camp (although I should forwarn you that the fact that he has Aspergers might raise some complications. Most of the camp activities involve a lot of teamwork between the kids, which I would think might be more difficult for a kid with Aspergers. If they don’t think they can fully meet your child’s special needs, they might require you to provide an outside aide. I’m not sure what the camp’s current policy is on this, but Karen would certainly know.)
FOR THE RECORD: I am not, nor have I ever been an employee of LCSI (the company that makes Microworlds). I am not currently an employee of TIC Summer Camp, although I had a wonderful experience working there, as well as attending the camp as a camper when I was a kid.