Phew, thank you all, glad I don't (yet) seem to have a budding delinquent on my hands! Love the 'tiny tyrant' description; I shall definitely steal that one to describe DS when he isn't listening!
IWillOnlyEatBeans: I wish DS would go all coy and charming with strangers - that's so sweet. DS often glowers, or turns his face away in what looks like disgust (at which point I start making excuses, "Oh, you're SO shy, aren't you, DS?"). He also gets gender wrong, usually when it's women with short hair wearing trousers, i.e. often elderly ladies trying to engage with him - very embarrassing!
Mumtocuddlebundle: DS has come out with almost exactly that same phrase!
Missmap: that's really interesting, thank you for posting. Of all the things, this one I found more worrying (along with the increased babbling) as I thought maybe he'd lost the skill. Funnily enough, though, yesterday he spotted a puzzle I'd bought a while back and put out intending to give to charity (I went on a puzzle buying spree when he was in his puzzle phase, so now the house is full of the bloody things) and he picked it up, asking to do it. I said that I was going to give it away to another nice child, as DS doesn't really enjoy puzzles anymore. He didn't like that at all! He immediately asked me not to give it away and started doing it properly 
Jammofifi: very relieved I'm not alone with the increased babble/silly noises. DS is also more boisterous now, with "smash", "bash" and "crash" being among his more commonly-used words! Funnily enough though, he's also incredibly affectionate with me. He really loves cuddling up and snuggling right in and giving me kisses.
I think sticker-reward charts might be the way forward as well; he adores stickers.