Homeschooling is legal in all 50 states, but laws vary greatly from state to state. The first step is for you to know and understand the legal requirements for homeschooling where you live. Next, notify your school, in writing, that your son will not be returning to his classes. You might also want to contact a homeschool support or resource group in your area. Besides being a good source of information for legal or support issues, a homeschool group also provide opportunities for your son to meet other kids who homeschool.
There are many good books on homeschooling, and I suggest that you read at least one. The Homeschooling Book of Answers by Linda Dobson or The Complete Idiot's Guide to Homeschooling by Marsha Ransom are both very helpful. Be sure to visit our How to Homeschool information center for more resources and advice. Good luck!