i think that's dodgy, I'm not suggesting that there's no way it could be innocent, as someone has said, he may just be trying to relate to the child by doing what he used to enjoy doing. However, in my view it is completely inappropriate. No adult should encourage any child to bend over and pull down their pants, in public or private. As people have said, it could be percieved as grooming, but even if it's not I think it's a completely unnecessary thing to do. I would be appalled if I was in a park and witnessed a man and boy playing this game, I would also be concerned if my boyfriend started suggesting to his son that they play games involving baring their bottoms at eachother.
Is the childs mother aware of this? I'm not sure it should be town gossip as it sounds like it's becoming, that could be very damaging to the man who could well be completely innocent on the charge of grooming. I suggest the subject is mentioned to the mother and she suggests to her boyfriend that this game is inappropriate and she wanys it to stop. then just keeps a close eye on them. I really would advise you to stop discussing it with so many aquaintances, a mans reputation could be potentially unnecessarily ruined.