I'm wearing some Polo cashmere (blend) socks right now. They are excellent - even below the calf they stay up pretty well, and they've held up quite well through quite a few washings/wearings. However, I wouldn't pay a whole lot for them, because I can tell they won't last as long as wool socks would. I got mine at Marshalls or somewhere like that for under $10 on clearance (probably more like $5-6) and I consider that a good deal.
All that said, they are very warm and occasionally too warm to wear (sometimes they get so warm my feet sweat and then get uncomfortably chilled). If you're in warmer weather it might be better to find him some good quality over the calf merino wool socks, which do well in warm and cold weather, and will last longer as well.