One of the simplest reasons a cham starts closing an eye is dehydration. If an eye was irritated or slightly injured such as scratched by a leaf tip or a twig he may close it too. More involved reasons can include a vitamin A deficiency or a sinus infection. Some questions...
Is the eye "stuck" shut especially in the mornings? Any colored discharge or buildup on the eyelid? Puffier than the other eye? Points more to an injury or infection. A vet can examine the eye close up to check this. You may need an occular antibiotic to treat the infection.
What's the humidity level in the cage and how dry does it get? If you are using an AC during the hot summer this can dry the cage out more quickly than at other times of year. Check it with a good quality digital gauge, not the cheapo dial gauges pet shops tend to have. Their eyes can be irritated in drier air, so it can respond well to raising the humidity or providing more time to clean out his eyes when you mist the cage. Misting gently for a longer time instead of shorter more intense blasts of spray help. If the humidity is too low much of the time, raise it with more live plants, a misting system or adding a fogger.
What do you dust with, how often, and what do you gutload with? Nutritional deficiencies can show up in eye trouble. Be careful not to just add vitamin A casually, as its easy to overdose.