I started a parent run lego after school program 3 years ago and have worked with a large group of K-3rd graders each year. We have participated in Jr. FLL (6-9 years of age) the last two years, last year with 3 teams, and this year with one team. Our first year we built purchase lego technic sets with motors, they were a big hit. We did not participate in Jr. FLL at all, but mimicked the format with a poster and display at our school science fair.
The first thing you should know is that there are costs involved. Some estimates of the basic costs for a team of up to six children includes:
registration for first $25
registration for each expo $25 - $30
lego supplies at least $300 (there are kits to purchase to make it easy, also, additional legos are needed for the model)
A couple other suggestions:
1 - There are camps available for your son to get involved, check out St. Christopher and Benedictine High School which both have summer offerings. I am sure there are others.
2 - there are lego models you can purchase which your son can build to use a motor. they are under technic motor functions. one such model is the dump truck #8052. We used sets similar to these our first year where we built a ferris wheel and a crane using sets from Lego education web site.
We attended the expo on Saturday hoping to figure out how to get my son (almost 9 years old) in a Lego group, but had no luck. I've been trying for well over a year to find a group that he can join -- with no success. We live just north of Richmond, in Ashland Virginia. My son is crazy for Legos and would like to someday be an engineer. The only Lego club that operates through our school starts with 4th graders. He is a rising 3rd grader. All I keep hearing is to "start your own group," but I am not really capable of that (sorry, I haven't got a clue about building these things) and my husband is bogged down with his job.