Well in general people have the common sense/good manners to say "Can I/my daughter/son pet your dog?" in which case I say "Sorry, no but he's still in training" and people are generally fairly understanding. Of course I understand you saying they might take offense if I just said no but I always try to be as polite as possible, it would be fairly easy to pinpoint me and the dog around the neighbourhood if I was to make a name for myself as being rude.
The amount of people that don't bother to ask, given the size of him at just 10 months old, is actually shocking. I would be terrified to just walk up to a dog of his size and stick my hand out. Just last week 3 kids came from behind and one of the girls started petting him, she then said can i pet your dog? and i said you're already doing that but if you don't mind he's still in training so i'd prefer if you didn't, the girl continued to pet him and said if he bites me i'll get him put down, meantime her buddy is swinging out of his tail and when i suggested he stop he said well you should get his tail cut then.
I would like to add he is trained to sit if anyone approaches and sit if another dog is passing (as other dogs tend to go for him given his size) and he is very well behaved but that doesn't mean to say he'll always be 100% well behaved.