An organization that may help.
PFLAG, a parent/child organization for people who are gay or have other similar sexual issues, has chapters all over the place. If you click on, you may find a local org, or at least get the phone number for him.
They may be able to connect him with someone, who can give him good ideas or sources to help him come up with Plan B (the plan where he needs somewhere to stay if he gets kicked out...)
I'm so impressed that you didn't freak!
I agree 100% with what dlowan said. He needs an adult who knows about gay adolescent issues to talk to before doing this. He also needs to be prepared for nutty reactions that may improve after an initial eruption, or that DON'T improve. He needs a plan for the worst case scenario.
It wouldn't hurt for him to join a support group on the web, to cyber-talk with others who have gone through this---as long as he can cover his tracks and not be discovered through the computer by his parents.
There is an impressive web of organizations for gay people--I hope he takes advantage of them.
Tell him we support him, and to be proud of who he is--every aspect!!