
To be honest if he isn't making himself sore I would try to ignore it, or distract him with something else if your in dog training/play mode.
Part of the fun for him may be the attention it gets him. If you ignore it, then he may not bother so much, or if he does, well if it isn't harming him, let him, just not in front of the vicar or Mother in law, ;)
Perhaps put him out of the room your in when he does it, so he realises this will be a solitary pleasure if he insists on doing it.
I think putting vaseline on the area daily may at least prevent any soreness and ingestion of such a small amount won't harm him. Works well on the bitches that have had puppies and got sore.