D.H.
I don't know if this would work or be the best idea for a 20 mo.old, but if you really think he's cold, and not too hot, you could try using a hot water bottle near him, or maybe pre-warm his bed with it. It will cool off later, but it could help him get to sleep. You could try that for a while if you want, then sew some uncooked rice into some cloth and microwave it for a couple of minutes.
By the way, I couldn't sleep if I was too cold either, or too hungry!