MP, we had DD2 in her sleeping bag right up until spring of last year...so bang on 2 yo...although I noticed before then that she was getting a bit tidier about keeping her blanket on, so it could've been earlier. DD1 was a back sleeper and stayed tucked up cosy in her bed right from the beginning so *never* had a sleeping bag. So maybe try him under a blanket for a night or two and see how he goes?