Try a US Army issue poncho liner. It is thin, flexible, will keep you warm when it is wet and is in camoflage. Available from US Cavalry Store as well as PX.

I have used a pair of poncho liners, with a ground sheet, as my sole bedding at 10 F, so they work pretty well.

Of course, whether or not that would work for your Grandfather depends a lot on just what temps you are encountering.

Regards

GKT

Texas Declaration of Independence 1836 -The Indictment against the dictatorship, Para.16:"It has demanded us to deliver up our arms, which are essential to our defence, the rightful property of freemen, and formidable only to tyrannical governments."