They are the perfect thickness - not too thick that he needs to size up in shoe size, and not cheapy thin.  Very soft and they have plenty of stretch.  Seem to be relatively durable as well.

Edited to add:

After 9 months, these socks are still in fabulous shape.  Not a single sock has worn out!  I ordered size 8-9 and my 11 year old (size 7 1/2 M shoe) fits them great.  I don't see the need to go up to the next size, since I don't want them to be bulky in his shoes.  These also fit my 7 year old (size 3 shoe), but he mainly wears the ankle style socks (same brand, and just as awesome!).  I can't think of a single complaint about these socks.  They're PERFECT!