Socialising your dog is one of the most important things that you can do for him. But socialising isn't about meeting his mates at the park every day for a rumble (like a bunch of blokes meeting for drinks after work). He needs to meet new dogs on a regular basis. This means that he's being reminded of how to say hello appropriately (imagine the greetings that bunch of blokes will give each other compared to when they meet their potential new in-laws, for example - big difference, huh?). If they only know how to behave with their mates then there can be lots of problems when they meet dogs they don't know.
So, in a nutshell, make sure he knows how to rumble AND give 'handshakes'