If he liked the "fabric" that the needle size/yarn combination made, then you'll want to stay with the same size needles and increase the amount of stitches that you use to accommodate his foot shape and size. If the fabric was too firm/tight, then simply going up a needle size may be all you need to fit the sock to his foot. Also if they were tight around the toes you can change the rate of decreases as you knit the toe or make the foot longer before you start the taper. I frequently do that because I like a fairly broad toe with some taper but not that much. Socks are small enough so that you can experiment without having to rip out miles of stitches if you want to make changes.