Aha, but he does know how to self-settle. He's been doing it for weeks!
You are right, of course, about the kind but firm approach - that is what we're doing but could be moreso, iyswim. I guess we'd just got used to thinking of him as easygoing and pliable...he's just like my brother, you know. All sweetness and light and cheery smiles and silly fun, until he digs his heels in over something and then you are truly up against an immoveable object.
Anyway he's conked out now after an impressive tantrumming performance Refused his sleeping bag too, for the first time, so he's spending his first night on top of under a blanket.