The SIMS gams online are a mixed issue. First of all you have everyone from pre-teens to adults playing them, which means that language, and conversation that can (and often does) contain inappropriate themes for younger players.
However, the games themselves are fairly neutral (other than a select few whose games themes are intentially violent) and teach a lot of life skills.
There are other online games of that ilk specifically garnered towards kids. For instance, Toontown by disney, is a bit of a cross between the online questing games (i.e. World of Warcraft or Age of Conan) and SIMS, but it uses disney characters and you have the ability to control how much he can chat with other players and which players he can chat freely with if you choose. Also there is Webkinz, in which you have to purchase on of their products (stuffed animal, trading cards, or charms) in order to get a code and care for your pet online. The addition to it just being a pet care game is that you can interact with other friends who have these "pets" and play online games with them or with yourself. Many of the games are arcade style, but there are also many learning style games which makes it very parent friendly. Webkinz also offers a parent control that allows chat to be restricted to premade phrases or a pw that allows them to chat freely with certain friends you approve...
These are only two that are out there, I am sure there are some that may be more age appropriate for your son...but if you want to let him continue with the SIMS game, I would monitor the chats as he plays...or sit with him as he plays one night and decide for yourself if this is something you really want him exposed to or not.
I myself play World of Warcraft (a great way to chat with other adults while my husband is serving our country and I am home alone with the kids) There are a significant number of kids on, but as I tend to group with other adults, I usually do not have to watch my content or theirs too closely...but as a mom, when a child is grouped with us or in general chat with us, I tend to try to steer conversation towards much more appropriate themes. Not all adults are this conscientous, however.
I will also say, that I have noticed I can tell when a kid is playing unattended but their parent comes in...their demeanor in game changes considerably.
I am just saying these are all things to watch for in your son if you choose to let him continue in the SIMS games. Do I think they are inately bad or corrupting? No...but i do believe if you are going to allow him to play a game that other adults play as well, you should watch the content of the chats and his responses to such as he plays.
Good Luck!!! ;-)