He wore his sleepsack until he was 22 months old.
We got the X-Large one. He liked it becasue it was the only thing he knew. He slept in it every night of his life. Now we use a blanket (he's two). He kicks it off everynight and his feet get really cold. I've tried putting socks on him but he pulls them off right away. I just keep putting his blanket on him all night.
If your son will not wear the sack then I would put him in something with feet and I would put two layers on him. I have always wondered what you would do without the sack. Most of the advice I have heard was to put them in layers.
Hope you all stay toasy this winter