I really believe dealing with disorders like dry eyes should begin on the inside. Like taking Omega 3 fats like fish or flex oil. I'd also suggest using a humidifier, especailly this time of year when we need to turn our heat on and spend more time indoors. Even a vaporizer in your sons room would help from drying out the mucus membrane.
Things like cucumbers and wet tea bags can also provide moisture for the eyes. Many of the over-the-counter eye drops for this condition have chemicals in that can actually cause other problems. But I think just mixing a teaspoon of sea salt in a quart of warm water and soaking a wash rag in it then making a compress to cover your sons eyes when he's complaining of his eyes being dry might help.
I'd also try keeping him away from stairing at computer screens, texting and playing video games for awhile might ease the problems.