He is very vulnerable right now and I'm sure that he is scared, given the considerable prejudice against homosexuals in our culture. He may even fear being rejected and condemned by his family and friends.
Help him find a therapist who has a background in helping adolescents sort out and deal with their sexual orientation. It's important that you not present this as sending him to a therapist so he can "get cured." You also need professional help so you can assist your son.
Your son may receive some beneficial, free counseling at the Trevor Hotline (800-850-8078), which serves lesbian, gay, bisexual, and questioning youth under the age of 25. Their web site is thetrevorproject.org. You might wish to call PFLAG (Parents, Families, and Friends of Lesbians and Gays) at 212-467-8180 or visit their web site, email@example.com to find one of a chapter near you.
Your son has now experienced your reactions to his telling you that he thinks he is gay and he is probably both relieved and more frightened than ever. I'm sure you will use your love for your son to give him what he needs at this critical time in his life.