in my opinion the old 'lets sit down and talk about the birds and the bees' puts too much pressure on kids. i believe that when your son starts making comments, that's the time to jump right in and casually talk about facts.
my daughter was 9 years old and began asking me questions about what other kids were saying at school. most of the information she had received was incorrect and some quite funny. I explained to her the whole process of getting her menses, sex vs. making love and the physical process. I want her to feel comfortable talking about these emotions and actions freely and it's ok with me if she chooses to share this information with friends. Some parents could not (or some kids are not willing to) discuss sex with their kids so if she has facts she can feel confident when other kids start to discuss and she can possibly assist other friends who have questions as well.
my son on the other hand is extremely shy and although he's not heavy, his brother (who is all bone) has teased him so much he is embarrassed about his body. He will not outwardly talk about sex, and yet I will bring it up, because he's 13 years old and i believe he needs to know facts as well. When we watch a movie or an advertisement comes on and people are kissing- he gets squirmy, (interestingly enough he doesn't seem to get bothered when my husband and I kiss)...anyway, I let him know wow, it's such a great feeling to kiss someone you love....so anyway in my opinion, keep looking for the casual moments so it's not an uncomfortable and possibly unwanted lecture
good luck