Cub scouts starts first grade or 7 years old. Boy scouts starts at 6th grade or 12.
To find out what groups are available in your area, try:
http://www.bsa.scouting.org/
You can also check the white pages in your area for the council office for your community. Look under "boy scouts." There should be a council, which organizes the packs (cub scouting) or troops (boy scouting). They would be able to refer you to the local groups.
Your public school may or may not have information; I would imagine that the secretaries would be a bit too busy this time of year to looks stuff up. Scouting is so big in our area that kids from one school might belong to any of 4 or 5 different packs.
We haven't found that scouting interfers with school at all. Occasionally we have scheduling problems while the boys are playing sports.
One other thing, cub scouting is family oriented, so be prepared to participate. There is usually one pack meeting a month (all the dens and families), plus weekly Den meetings (the smaller groups broken out by age level). At cub level you or your spouse should plan on attending ALL of the meetings with your child as their adult partner. As the kids get older, less parental involvement is needed, but if your son is hyperactive, plan on staying to help as the leaders are just parent volunteers and can't run a meeting while trying to babysit. (Can you tell I've seen this in action??)
Scouting offers a great opportunity to the boys. It's made up of a lot of volunteers who do an incredible amount of work, and the boys and their families can really have a great time.