Usually the pattern says to knit for X inches then begin the shaping. If it doesn't, or you're making up a pattern, measure one of his sweaters and see how long from the underarm to the lower edge. It may help you measure your piece if you put part of the sts on another needle so you can spread it out flat, then put on a table to measure.
Definitely. Measure. Put his favorite sweater, the one that fits him best, on him, and measure the side from however long the sweater is to the bottom of the armsceye. Then measure this with the sweater off of him and lying flat, to see if there's any difference that you'll have to allow for in your current garment.
Do the same with the bottom of the neck measurement in the front, and then again in the back.
Measure the row gauge of your sweater and figure out how many rows you have to do before starting shaping, or just knit until the measurement is correct, allowing for any blocking, washing, or hanging out from wear, which your swatch (you made a swatch, right?) will tell you if there'll be any changes from any of those things.
ETA: do remember that if you're sewing a seam at the armscye, you need to remember to allow one stitch/row for a seam allowance!
OTN: Eyelet Chemise in Handmaiden Sea Silk (colorway: Midnight). Still. And a purple Donegal Tweed set of fingerless mitts, to try out my new Hiya Hiya interchangeable needles.
Latest FO: A shrug for an Anthropologie swap in beautiful Casbah sock yarn, in Cedar, a dark green semi-solid, my own pattern. Also a quick Noro Silk Garden neckwarmer for my friend Aideen, in a vine lace pattern.
Or look at other Aran sweater patterns-- free ones on Drops, Lionbrand or knittingpatterncentral.com, and see what they suggest for doing from the bottom up to the armhole. If you look at a number of patterns, you'll probably find a measurement starting to seem pretty common.