It's very normal for dogs to become more discerning about who they socialize with and less interested in playing with every possible strange dog and human as they mature. Not too different from us, really, if you think about the difference between how a group of 4-year-olds who don't know each other interact compared to 14-year-olds! I wouldn't be concerned at all if he just seems like he's less interested in being everyone's instant friend. If you start noticing he's less interested in participating in other activities he enjoys, it might be worth a vet visit to check for underlying pain or other medical issues, but from your description it's most likely just a case of your boy growing up.