Dawn,
I made these, using a basic sock recipe, for my mom. She loves them and told me I can’t quit knitting as long as she’s alive, so she can have her slipper socks LOL
Any worsted weight yarn will do, as will any bulky yarn that is the a similar thickness to two strands of worsted weight. I made these with Simply Soft, simply because it’s what my mom prefers.
I have tried to convert her to other yarns, since I prefer to knit with natural fibers. I knit a pair from some great bulky wool/nylon blend that I reclaimed from an Eddie Bauer sweater that I had used to knit some hiking socks for my husband (and he loved), and I knit her a pair from some Jaeger Matchmaker superwash merino wool, and she still prefered the Caron Simply Soft, so Caron Simply Soft it is
When I knit these for my daughter’s, I use wool or wool blends, but it’s often yarn reclaimed from bulky sweaters, so I can’t give you a specific brand.
I made these, using a basic sock recipe, for my mom. She loves them and told me I can’t quit knitting as long as she’s alive, so she can have her slipper socks LOL. At least I know my knitting is appreciated 
A few things I have discovered. My mom wears these everyday anytime she’s not out of the house, so I have watched a bit of wear.
Most yarns won’t hold up for a longtime to super duper heavy use (walking all over the house all day) with this pattern, because they aren’t knit really tight like a regular sock. However some do better than others. The plus is that I can knit about 5 or 6 pairs of these in the time it takes me to make 1 pair of regular socks, so I replace my mom’s as she needs them.
Softer wools (like merinos) don’t hold up in the foot very well. I won’t use the Jaeger again for slippers. Also, yarns with minimal twist tend to not hold up as well. If those are going to be used, you might want to put either the anti-slip coating on or a leather sole to help them last.
If using wool, something with a bit of nylon mixed in can help the foot of the sock hold up, and make it more likely that it can be washed without felting/shrinking.
Leather soles made for slippers can make these last a long time, but can be a bit pricey. I know some people watch their thrift stores for leather skirts and such, and cut a leather bottom patch out of those. However, I don’t think you can machine wash with the leather bottoms.
One last thing… if your FIL has more than a narrow foot, I would recommend a few more stitches cast on. 34 for a narrow or medium man’s foot, and 36 for a wide man’s foot. I just wrote the pattern using these numbers up for someone else, but haven’t tested them to make sure I didn’t miss a number change/make a mistake. Let me know if you need those changes.
Mama Bear