Some of the best resources for starting a business are NOLO's legal small business books. I recommend them to anyone who wants to start freelancing or their own business (I actually do both by owning a PR firm and doing freelance writing / editing on the side).
www.nolo.com
I also run a Consulting & Freelance site that might have some information that could help you. You can find it at
http://consulting.allinfoabout.com.
I can't give you specific info on Florida's rules, but generally you need to register a new business with your state's revenue office (usually free to do as long as your business name includes at least your full last name). You should get a sales tax license (also should be free in most places), just in case you decide to sell anything that requires charging sales tax. Then get a federal EIN from the IRS (also free and can be done over the phone), so you don't have to use your SSN for everything (better to keep that private).
And remember, freelancing is NOT the same thing as starting a business. If everything's in your son's name, but you're doing work too, then he would be a business owner, and you would be an independent contractor working for him, and should have a contract stating so. You'll each need to pay your own taxes separately still. Payments go to him, then he pays you and records it. If you're BOTH freelancers, you'd essentially be his sub-contractor, and need a contract and payment system that can be recorded as well, or your son's going to have a heck of a time with taxes.
Jenn