If you really
want to put off doing the ribbing until later, you'd have to do a provisional cast on, whether that be the kind seen on this site or the kind that uses a crochet hook. I've done both kinds -- the one Amy shows on this site is faster and if you use a spare circular needle to do it (like she shows at the end of the video), you can knit the ribbing immediately. If you do one of the crochet hook methods, you can't knit from it right away and have to pull out the waste yarn stitch by stitch to put a needle through the live stitches. [IMHO, the only reason to really bother using one of the crochet hook methods is for a hem that will be sewn to the inside of a garment stitch by stitch.]
Which cast on method are you using? Some can look better than others regardless of effort. My longtail CO and cable CO look the most even, but that's just me.
If you like the cable CO, you could do the ribbing version shown on this site. Longtail CO would be my choice, though, since it has a little more stretch -- which you'd want in the hip area.
My final thoughts are that you should just do the ribbing first. You could measure your dad from underarm to where you want the sweater to end. If it's a surprise gift, you or an accomplice could sneak into his closet and measure a vest or sweater that is the desired length and go by that measurement.
'Sorry, longwinded as usual!
Good luck and have fun!