Did you try Lion Brand
Any hat knit in the round you can knit in flat as well. This is just a very simply hat and you definitely don't have to add the pom pom. Just remember that when you knit stockinette in the round you just knit rather than knitting the RS and purling the WS - Sooooo when knitting flat you simply cast on the number of stitches needed (or called for) and then work in stockinette as you would normally on straight needles for the number of inches stated. The end of a round will be the end of a row for you. Don't forget when the decrease rows start that when they say "round 2: knit" you really want to purl. Do you get what I mean.
If you choose this hat you would probably want to knit about 7 inches before decreasing if he wants to fold it up. You also want to play with the gauge to fit his head. Here's a formula for that:
So I just linked to one particular easy pattern in bulky - but you can use that formula and use any pattern you would like - or just wing it
ETA: I should probably say most hats knit in the round can be knit in flat. I have a bad habit of using words that imply absolutes when I should really be using words that imply a general standard! Sorry! :^)