The part of this that makes me feel a bit uncomfortable is the pecking. My daughter is 12, almost 13, and as yet has never kissed a boy. She and I are ridiculously close, and she would tell me, would feel compelled to tell me, if she did.
Growing up my sister always had boyfriends. That started for her in kindergarten. I had a first grade boyfriend, who brought me flowers, but I didn't like him, and everyone teased me about him! That didn't last, and didn't have a boyfriend again for a long time. My sister, on the other hand, always had a boyfriend. Her "relationships" were cute and innocent. Honestly, she probably didn't start kissing her boyfriends until she was older than your son and his girlfriend. Still, in my opinion, what these young relationships do is open some doors a little earlier. My sister had sex before I did (I'm the oldest) and she had her first pregnancy years before I did - as a teen. This isn't because she was generally an irresponsible girl, and she wasn't promiscuous. Her first partner was also the father, who eventually became her husband. She had my niece, graduated from high school at the top of her class, and was the stand out student in her program at Temple University. She's an executive at Independence Blue Cross now and works with teenage mom's to encourage them, ensure that they know that a future is still possible. I just wonder, if she would have gotten her first kiss later in life, had she been a sweet 16 who's never been kissed like I was, would she have become sexually active a little later? A little later would have enabled her to have finished her senior year in high school instead of her junior year when she had her first child. My question to you would simply be, is it time to open that door?
Fyi, my mother was also an open mother, the kind we could talk to. She wasn't unnecessarily strict, she wasn't distrustful. My sister did marry my niece's father, and my mother's assessment of him being a good guy and coming from a good home was right on. But didn't things happen a little too soon to these two good kids?
Maybe you could encourage him to call her and treat her as his friend, and encourage him to treat her special because she is special, but maybe you'll consider telling him that he's young to have a girlfriend, and that kissing isn't appropriate for him now. You can tell him that you didn't say anything for a while because you know that he means it in the right way, but that just like most things, there's a time and place for everything. For instance, you can't get your driver's license until you're 16 (in PA), you can't drink legally until you're 21. Driving isn't wrong or bad, nor is a little wine or beer, but it's wrong from you when you're too young. Then you could nurture the aspects of the relationship that you think are contributing to his development, like how to treat a girl or woman in your life - be it a friend, girlfriend, mother, or sister - and that it's great to have friends of both genders. That's a great one for him to get a handle on at this age when most boys don't know how to be friend with girls, and vice versa. You can include her in functions as you would his best boy buddy. The things he's doing are not just like two kids playing. They are acting like a couple. It sounds to me like what I'd expect to be happening in my daughter's life maybe in a year or two - although she is quite conservative. She's interested in love, and it's in all the teen shows and movies, but she's also not interested in the pressure that can come with that, or the distractions.
I graduated high school a virgin, and was 25 when I had my daughter. I mention that as a foil to my sister's experience. Hope all my sharing helps!