There is a group based in San Francisco called San Francisco Sex Information, who you could talk to, and they also have a vast database to provide therapy referrals. I took their training program last spring.
They have a web site at http://www.sfsi.org
You can call their switchboard or send them an email.
If you can get your son psychological counseling now, you can stave off all sorts of problems. The part of abuse that usually does the most damage isn't so much the sex part as the power/control part. As kids enter puberty and start to gain control over their sense of self and place in society, their viewpoints and methods can be heavily skewed by abuse when they were younger.