If I look back, my youth group in the mid 70s spawned a lot of couples. That wasn't the focus of the group by any means, but it was also a safe place to meet people. There was only one couple that pushed the envelope, showing more affection than I was used to in public. They married a few years later and I think are now divorced. Such is the life of teenage "love".
Personally, I'm not big on the public display of affection. (I had to laugh...I had to look up what PDA meant on an online slang site. I used to think PDA was a personal digital assistant...like a palm pilot...but I knew that wasn't what people were talking about. Go figure!) I may not be up on the latest acronym, but I'm also not a prude. I don't mind seeing people walking hand-in-hand or arm-in-arm or thoroughly enjoying each other's company. But people slobbering all over each other in public, whether young or old, just isn't cool in my book.
If the kids are truly making out in all the places your son mentions, think of it as an excellent opportunity to see what your son thinks about it and for you to express your feelings about it. It's an excellent teaching point and springboard to talk about attraction and PDA (see...I can be taught! : P).
But personally, I think a conversation with your pastor would be appropriate. It doesn't have to be a big judgmental "did you know" kind of conversation. But it might open the door to talking about different ways that people handle things like this.
There's plenty of time for kids to grow up. We don't need to rush our kids to that path. Perhaps the pastor could weave the topic into some youth group discussions to explore the comfort and discomfort various kids have with the topic. What better place to talk about it than in our churches and youth groups?
I'd rather not see people clinging to each other in public like Velcro. But at 13, it's a fine line. We want to teach our children what love and respect are all about. We want them to learn affection and joy. But it's not appropriate to "get a room" at that age. Still, there is a place and time for everything, and I think these two kids could be asked to cool it a little.
There are LOTS of ways of expressing one's feelings for each other. Sure, kissing and hugging feel really nice. But talking and doing stuff together and with others can enrich their lives as well. It can be an exercise in learning a wider range of ways to enjoy each other. : )