He isn't going to spend lots of time around dogs, the thread is about my child going to a friend's house - I don't think a visitor should have to watch to this extent, the dog should be watched by the owner, but obviously I don't know if that will happen.
But if your son is 13, the parents may not be home, and if they are, they certainly aren't going to be in the same room as visiting teenagers for the entire time, or supervise the dog constantly either - and I don't think they should be expected to.
Again, it's not about being constantly vigilant, just vaguely aware of what the signs are and what to look out for if the dog is in the room and does approach him.
Regarding dogs in the family, we just avoid them. If you don't interact with the dogs and if you are not in their space, how can you watch what they are doing anyway?
But life should tell you that you can't always avoid dogs or expect them to avoid you, so IMO it's just common sense to teach your child how to behave around dogs (or any animal) and what to watch out for - just in case - to give them the best chance of staying safe if there's an issue.
I think it is unreasonable/unrealistic to expect a visiting child to be scrutinising the dog's ears/eyes/mouth tbh.
At 13, you need to be realistic - parents won't always be around and it's pretty normal to leave a teenager in sole charge of a dog, so knowing basic body language is never going to be a bad thing. I never said they need to sit there and constantly monitor the dog, lol.