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!