Unless his head is more than 24 inches around, you won't be able to use a 24-inch circular needle. 16 is usually the best length for hats since you can scrunch stitches on a needle, but not stretch them beyond their reach.
At some point, when your decreases prevent the stitches from fitting around the circular needle, you'll have to switch to double-pointed needles.
You could also try using magic loop--Amy has video on it. It's a way to use a longer circular needle for smaller projects.
To soften the cord, you can try dipping it briefly in boiling water and hold it straight to dry. Most needles that you buy in craft stores have stiffer cables. I think that's why so may people like Addi Turbo or Knit Picks needles. The cord is very flexible.