This is a little bit of a hi jack but I think it will help you more than socks.
Instead of socks, I would look at boot gloves. They're incredible and about the price of two pairs of socks. It's a layer of thick neoprene that acts as a koozi for you boots. And just like a thermos, put in cold, it stays cold. Put in warm, it stays warm.
My keep feet warm trick is to put hand warmers in my boots when I leave the house for the mountain. I shove a pair of sock in the cuff to keep the heat in. When I get to the mountain and put my boots on, hand warmers go in the gloves, feet in the boots and boot gloves over the boots. Nothing wrong with kids starting out with very warm boots.
I also keep the boots in the car and not the trunk or cargo box. They are transported inside in a bag and not thrown over my shoulder.
I struggled for a season to keep my daughter's feet warm and the boot gloves did the trick. She thought they were too dorky looking until I made her try them anyway. Now she makes sure they're in her bag.
My only issue with them is the strap underneath will eventually rip if you walk on hard surfaces. I saw that mine were starting to fray so out came the duct tape and problem solved. You also need to make sure they aren't interfering with the toe bindings.
Don't forget about the hand dryer in the bathroom as a booster shot for the boots and feet.
When you start getting cold feet, go inside and put on warm socks and start over. You're son might need to take more breaks than the rest of you.
Hope this helps. Cold feet can suck the fun out of skiing pretty quick.