Caroline wrote:I'd give him a duvet, he'll soon work it out

WSS but a couple of big cellular blankets instead so he can layer up as he likes. Id get a few fleece sleepsuits also so he is less likely to get cold (primark do them £4 each) if he does kick them off.
DD is 19 months and doesnt have sleeping bags anymore. I make sure she is well dressed (longies, long sleeved top and nightie to cover her back ATM) and she has a big cellular blanket which ive been telling her what its for and getting her used to it (she used to hate blankets), she now sleeps well with it and i will add a few more blankets when it gets cold