First, I think it's important to understand this is a very exciting time in your son's life. He's just figuring out girls aren't really all that yucky. Obviously, you can't tell him he isn't allowed to like the girl. He can't just flip a switch that prevents the attraction. Even if he could, I assure you there will be other girls in the months and years to come. While you don't want to rob him of his momentary joy, you don't want to encourage the relationship either.
 
It sounds like you're doing a really good job of setting guidelines for dating and monitoring his email and Facebook. If he has a cell phone, you might also make sure he doesn't keep the phone in his room overnight. Establish a rule where all cell phones are required to be in the kitchen charging by a designated time each night and cannot be retrieved until a designated time the next morning. This will prevent calls or texting throughout the night which is common for many teens today. 
 
There are also ways to redirect his focus from the girl. Maybe there's a hobby or something he enjoys doing.  If he loves basketball, keep him busy working on improving his game - or maybe he's into music, paintball or Scouts. Whatever he's interested in, keep him busy and focused on those activities.  Remember, bored kids are far more prone to engage in sex and drugs than those focused on fulfilling an exciting dream.
 
If you haven't discussed your stand on abstinence, it's time to do that. Make sure your son understands your family values. "No sex until . . ." and then you fill in the blank. In other words, when will you be okay with him having sex? This is not a one-time conversation. He needs to hear this over and over through the coming years.
 
Keep reminding your son how wonderful he is and what an amazing future he has. Find opportunities to remind him how sad it is when dreams are derailed because of an all-consuming relationship or unplanned pregnancy.  Talk about how far is too far. He needs to know where you expect him to stop when he eventually gets to be alone with a girl. It's far better to instill these values now instead of waiting until he's 16 and in a serious relationship. Despite the fact that roles are now shifting where girls are becoming the aggressor in many relationships, help your son understand you expect him to treat all girls with respect and not as something to be used for personal pleasure.
 
If your son is into sports, use Tim Tebow as a great role model. This guy was the quarterback for the University of Florida for the past several years. Not only did he lead his team to two national championships, he also won the Heisman Trophy and was featured in Sports Illustrated last summer. But here's a little known secret about Tim Tebow... he's a virgin! That's right! This guy is committed to saving himself for his future wife.
 
Perhaps your dinner time conversation some evening could focus on the boundaries Tim Tebow might have set when he was in middle school to stay focused on his football career. Then keep watching the news for other role models and teachable moments. Oh, and if you have a copy of my book Teens Sex and Choices and you haven't gone through it together already - now is the time! The book covers all these topics in detail. The Family Discussion Questions are designed specifically for you to explain your values. Remember, serious relationships and raging hormones are inevitable, and now is the time to instill your values. 
 
More than anything, I advise you not to over react to your son's new attraction to girls. You've got your guidelines in place and you're monitoring where you can. Now just relax and let your son know you love him unconditionally and trust him to make good choices. When he does make mistakes along the way, don't rush to his side to fix the problems. But at the same time, don't turn your back on him. Ask him questions like, "How do you plan to resolve this problem?" Allow your son to learn from his mistakes by paying the necessary price. Making mistakes and learning from those mistakes is what will allow him to grow into a mature responsible young man. What better time than now to begin learning that choices have consequences.