I like your take on Jimmy a lot. This is how He SHOULD be written. He's boring if you just make him another adult. My concept of Jimmy would make him a skateboarder with an Ipod going all the time. He would have great computer skills and border on being a Hacker. And, though some people here hated it, I liked it a lot on L&C when Jimmy would call Clark 'CK'. It's what a kid like that would do. He'd have his own names for everyone. That would be right in line with why he would call Perry 'Chief'.
Oh, and the Bow Tie would definitely HAVE TO GO!
When Jimmy was created back in the 40's, bow ties were common to teenagers. It isn't anymore and is just a cliche now. The Geek band t-shirts are a perfect touch. Maybe even have him wear it over a long sleeve T. He'd wear Luggs instead of shoes. He'd have a state of the art digital camera around his neck most of the time.