Your son's socks are hard and crusty as a result of them being overly soiled and dirty and because the dirt and soil was not completely removed from the socks during the laundering or if you have hard water that causes a buildup in the socks fabric fibers.
Sweat contains salt, which crystalizes when it dries, making the socks hard.
It can also dissolve chemicals or existing dirt from inside the shoes.
I suggest you first try washing the socks as you would wash any laundry, then rinsing them thoroughly in a lot of water to remove the salt and any remaining chemicals.
Wet socks are so uncomfortable because the water seeps through the socks and into your skin and the skin absorbs the water and causes your feet to become waterlogged, wrinkly and sometimes smelly.
The longer your feet are wet and your socks are wet the more damage to your skin and blisters and painful cracks can form.
Wetness can increase the friction between the foot and the sock, and if it's not taken care of, it can lead to sores and blisters on your feet.
To whiten your socks with baking soda, soak your dirty socks in a tub of warm water mixed with baking soda.
Leave them for a few hours before washing them in the laundry with your regular detergent mixed with a little more baking soda.
You should wash socks twice if they are really dirty, although washing socks once is most often good enough to get them clean and get rid of bacteria and fungus.
Socks should be washed after each use.
When you wear socks you can get sweat and bacteria in the socks and wearing the same socks longer than a day or not washing the socks between each use can cause your feet to get bacteria and fungus and even rashes on your feet.
Wearing dirty socks even though they may seem clean can also make your feet smell and your feet hurt.
Even if your feet aren't sweaty, you shouldn't really wear the same pair of socks two days in a row.
The reason being, is your feet contain around 250,000 sweat glands and can excrete as much as half a pint of moisture a day.
Bacteria can thrive on dirty feet and grows rapidly in warm, damp socks.
A bacterial infection can lead to painful wounds, possibly even be life-threatening.
Typically, socks can last between six months to a year if worn once a week.
Socks should be changed regularly to avoid unpleasant smells and athlete's foot (a fungal infection).
Fungus loves warm, damp and dark environments which makes your socks the ideal breeding place.
So you should change your socks at least once per day or more often if your feet are sweating a lot or they get wet.
Wearing socks to sleep comes with very few risks, other than discomfort for those that prefer the feeling of being barefoot.
Be sure to wear socks that are loose, and do not wear compression socks to bed unless directed by a doctor or other medical professional.