I am assuming you have a basic pattern but are not sure how that compares (and therefore how many stitches to cast on) with the yarn you have.
I would knit a swatch with the yarn and needles you have to get the gauge. (cast on twenty or so, knit for a few inches then measure one inch in the middle, count the number of stitches and rows in the inch, this is the guage).
then i would measure an item of clothing that he already owns that is a similar size and fit to what you want to knit. You then multiply this number buy the guage to get the number of stitches you will need.
So if you guage is 4 stitches per inch and you meause the circumfrence of a sweater as 30 inches then you would want to cast on 120 (4 x 30) stitches to get the right circumfrence for the sweater, if knit in the round, or 60 for the front and 60 for the back of knit flat.