No specific legislation for sat-navs, but as suggested previously, there are other offences that would easily fit the bill if you were seen doing what you describe whilst driving.
Let's be honest- if you're typing in a destination as you drive along, you are going to have your eyes and thoughts...
Jeez. GPS trackers on vehicles aren't actually that accurate, and often update positions and speeds at fixed intervals (say every thirty seconds) rather than constantly. So there's actually no way they could tell if your son was driving or not, not to the nearest few seconds anyway. And what if...
How many times do you think they hear the "Oh, I was just rejecting a call", or "I was just turning it off", or "I was just [anything else]", though?
For what it's worth, I had somebody tell me once "Oh, it's OK: I wasn't using the phone. I was just sending an email" :facepalm: :lol:
No, because you're not using a sat-nav to perform an interactive communication function, such as calling, texting, using the internet. It's just telling you which way to go. It's not covered by the mobile phone legislation.
However, if I saw you driving whilst clearly being distracted by...