N.
"First love" is the toughest for them to get over. The only thing you can do is remind your son that there are other girls out there, and that maybe some day he and this girl will "find" themselves together, but for right now the timing isn't "right". Remind him that they are both young, and should enjoy their teenage years. If they truly love each other then maybe someday things will work out for them, but right now they need to take the time to "experience" life and "make sure" that they do truly love each other. Be supportive and try to find other ways to distract him from thinking about her. Good luck!