Get some plain saline solution and wash your chams eyes a couple times a day. Just drip the saline on his eye and allow him to wash it well. Try that before a vet visit. jmo
What species of Chameleon do you have? You also don't mention if he is listless and not eating.
There could be a wide range of causes for this symptom.
Some rule outs would include eye infection, foreign material in the eye, retro bulbar abscess, dehydration, parasitic infections, temporal gland infection (in species that have them) and metabolic disorders.
First thing to do is hydrate him. Put him on a plant and lightly shower him with lukewarm water for a good 30 minutes. After that, if his eye is still close and appears matted then use a warm very wet wash cloth to warm compress the eye. Do this for about 5 minutes.
If that doesn't get his eye open or if he is not eating or listless then you need to get him to a good reptile veterinarian ASAP.