That LEO pattern is great, and the one I picked out to knit for DS for Xmas. He was very specific that it NOT have cables (I made both his sisters Aran sweaters) or color work. :(
I haven't experimented with yarn yet, but from the gauge it looks like a DK or sport weight would probably work. Size 3 and 4 needles = LOTS of stitches.
I'll probably use a wool/cotton blend (superwash if I can find one I like, because a college freshman throws everything in the washer and dryer), or Woolease sport which washes and dries very well.
I wouldn't do this sweater in 100% cotton because cotton doesn't have enough 'bounce' and it's likely to pull out of shape. You could go with an acrylic with some wool or cotton in it. There are some that are very soft.
A lot of the yarn manufacturer's websites ie: bernat, lion, paton, plymouth have a yarn selector tool where you can select a gauge and it will give you a list of yarns which knit to that gauge.
If washability isn't an issue then there's SO many choices, but to keep the cost down I'd go with something from Knitpicks. Get KellyK to feel her color cards for you :rofling:
Let us know what you decide on
Happy Knitting :XY: :XY: :XY: