As a parent, you still have a lot of positive influence in the life of your pre-teen. Take advantage of it. If you have fallen short in this (or any) area of communication, take the opportunity to dive in now.
-- Spend time with your child. Most often, in those moments together, while driving in the car, cooking dinner, and just relaxing, your child will open up. Give your child that opportunity.
-- Praise your pre-teen's accomplishments, his talents and his character. Your child needs to know just how much he is cared for. Feeling accepted for just the person he is frees him to explore his own personality and life values.
-- It is important that your pre-teens understand the normality of human sexuality. Often at this age, with their bombardment from TV, music and magazines, they may feel that sex is "weird" or "perverted". Help your child to see that sexuality is not (and it not intended to be) what is usually represented on TV and in the media.
-- Let you child know that their own sexual feelings are normal. You might want to share how you remember feeling at their age. Being honest about yourself and your feelings helps them to better understand themselves and express their feelings.
-- Talk to your child about masturbation. Explain that pleasuring himself is normal and healthy. Though many people choose to masturbate, others may not feel comfortable with it.
-- Sex is intended to be a pleasurable activity, and not just for reproductive purposes. It is part of a committed, loving relationship.