We will need more info in regards to your setup and husbandry. Please take time to fill out the ask for help sticky which is one of the first threads in the health forum.
It may possibly be something stuck in the eye, since only one eye is closed. Have you tried a saline rinse?
Chameleon= Mellers 1 month had him for 3 days
HAndling= Alomost none
Crickets= 4-6 small crickets
Supps= Calcium with d3 twice a week
Mist= 3 times a day for like 5 min
poop= once and it was normal looking
cage= 18x18x24
lights= 50 watt day light and reptisun 5.0 uvb
Chameleon= Mellers 1 month had him for 3 days
HAndling= Alomost none
Crickets= 4-6 small crickets Supps= Calcium with d3 twice a week
Mist= 3 times a day for like 5 min
poop= once and it was normal looking
cage= 18x18x24
lights= 50 watt day light and reptisun 5.0 uvb
Make sure to double check with someone about giving D3 twice a week. Hopefully someone with a Mellers with chime in on it. A lot of times an out of alignment bulb can output too much uvb causing eye closure. It usually seems to be one eye when that happens. However other issues could be the cause. Try a uvb meter on the bulb. If it is working then get to a vet for the correct treatment. Even if bulb is working, a vet visit could only be positive. Especially if he is only one month old. Good luck. By the way, they get HUGE!
Twice a week for the D3 is too much. This should be given twice a month. The eye issue could be and is most likely a result of too much D3 IMO. Make sure he is hydrated. Get some silkworms or hornworm ASAP to get his system flushed. Make sure he is drinking well. Hydration is key. Hope this helps.