The appropriate sock size for a 10-year-old boy will vary depending on the child's foot length, but generally boys around this age will fit into ladies' socks with the correlated shoe size between 5 and 10. To be sure, measure the child's foot length and use a sizing chart to determine the correct sock size.
The Sock Drawer is a sock retailer located in San Luis Obispo that provides a sizing chart for both sock and shoe sizes for kids, men and women, that is available on its website. Other retailers such as the Craft Yarn Council provide comprehensive sizing charts for crew-style socks and include measurements of the foot circumference to determine an appropriate size. These and similar resources are available online.