Wow that seems beyond the normal experimentation or curiosity, someone taught them how to do that or they saw it somewhere. One of them might have been molested or allowed to watch inappropriate movies/videos and saw that. I don't think kids that age normally think to put their mouths down there. I would definately talk to a professional to make sure that no one is abusing them in any way.
There is concern beyond the normal harnless curiosity and exploration for the age because of the oral nature of the behavior. It is not within the normal spectrum for children to engage in oral sexual behavior. So it is definitely sensible to speak further with your son (and hopefully his friend's parents will do the same) about how the idea to do this came to light. It would not usually occur to children to do this. It is likely that one of the boys experienced similar behavior, either by direct involvement, by wirnessing it in person, by viewing it in a video, movie, picture, etc.