I don't disagree necessarily, but this is way too subtle for a kid, so it's not a practical answer.
Besides, as a semi-professional linguist, I must say you are confusing semantics (e.g. your boxes example) with pragmatics which is what we are talking about, where one uses words to mean something other than what the dictionary + propositional logic say they mean. These are often very confusing because they rely on cultural context and both kids and foreigners often screw up when they deal with them.
I don't disagree necessarily, but this is way too subtle for a kid
Too subtle? This is just one tiny part of growing up and learning to interact with other people. If it's too subtle for him at this point in his development, then you'll just have to wait.
so it's not a practical answer
It's a practical answer in that it shows why you shouldn't encourage him to respond like that in those situations. I would have to know a lot more about your kid (and perhaps also way more about parenting) to know whether you should try to discourage it (and how to go ab...
I've seen an article on LW about Santa Claus and most people were very keen on not lying to their kids (and I agree). I have a little kid who is generally quite truthful, innocent enough not to lie in most cases. I noticed recently that when someone asks him, "How are you", he usually answers in detail because, well, you asked, didn't you? When I was a teenager I hated people who lied and I tended to ignore these unwritten social rules to the extent I could. I.e. I didn't ask if I didn't want to know and people thought I was rude. So, my question is, should I teach him to lie upon these occasions?
More broadly, I was thinking, why am I committed to being truthful, in general? I guess because I would hate to be lied to myself. This is a kind of magical thinking maybe, or maybe it's a part of the social contract. This sort of lying in fact promotes the social well-being because to answer truthfully creates an unwelcome burden on my interlocutor who asked out of politeness and is not in truth interested. But it still feels wrong to lie. Even more wrong to teach your kid to do so.