Thanks for the input.
I'm using a WWI pattern that was designed for wearing under a helmet, so, hopefully I'm okay there.
I found some nice soft yarn (Red Heart Soft Yarn). It's not thin, but it isn't too bulky, either.
The good news is that it's knitting up quickly. I was afraid this would turn into a major project, but I'll have it finished in just a few days and it only cost $2.79, so if it doesn't work out, it's no great loss. I can also use it as a prototype. If our son wants a thinner one, or other modifications, I can make him another one for Christmas.
BTW, I used the "cast on in pattern" for the ribbing and I'm thrilled with it. It's a pain, compared to regular casting on, but it really makes a difference in the way the ribbing looks.