Could be anything. Old injury, bug bite, abscess…
I'd just keep an eye on it. If it gets bigger, go to a vet.
See how the eye is sunken in and the casque is concave? This cham is dehydrated and skinny. Do you know about misting, drip systems and humidity? I'd mist him twice and hour for a while. Get him rehydrated. You can also inject water into his bugs with an insulin syringe to get a little more water into him ASAP.
Sad. Glad you saved him.The person I got him from kept him dry and without UVB. The enclosure is super dusty and smells like filth. Thank you for the advice. I’ll get him cared for and situated tomorrow.