Boys become interested in girls at various ages. It really depends upon "who" your child is. Some are late bloomers and become interested in girls in their late teens.
I've found some books that you might find useful. Look over these at your local bookstore to see if these help:
1.Our Boys Speak: Adolescent Boys Write About Their Inner Lives
2.Everything Guide to Raising Adolescent Boys: An essential guide to bringing up happy, healthy boys in today's world (Everything: Parenting and Family)
3.Your Adolescent: Emotional, Behavioral, and Cognitive Development from Early Adolescence Through the Teen Years David Pruitt
Many young teenage boys like the series of books by Jerry Spinelli: Space Station Seventh Grade, Jason and Marceline, Maniac Magee. These fiction books deal with a variety of teenage issues.
Consider scheduling appointments with your child's pediatrician and with his school counselor. Both would have good developmental information to share and could discuss any concerns you might have.
Hope this helps to get you started in answering questions about your son.