That definitely looks like a very bad burn. He probably is too cold and was sitting too close to his heat lamp. Chameleons not feel heat the same way humans do. They can burn themselves trying to get their inside body temperature warm without realizing it.
Do you measure your temperatures? The hottest place in his cage should be no more than 28 C. The rest of the cage can be 21-23 C. Jackson's chameleons do not need to be as hot as other types of chameleons, but they do like a time to warm up, particularly in the mornings. My Jackson's does bask a lot in the winter - more than in the summer, even though the temperatures in our house are about the same. My Jackson's also has a significant appetite loss during the winter. I think it is related to overall lower temperatures and humidity and shorter daylight hours.
Can you fill this form out? Take your time and give as much information as you can.
https://www.chameleonforums.com/how-ask-help-66/
Right now, you need to adjust his basking spot. Either lower the branch he sits on, or raise the lamp up. This will prevent him from continuing to burn his back. Do you have a veterinarian? The burn may need an antibacterial cream like neosporin or silvadene. You may need a prescription from an animal doctor. That is the usual way of treating burns.
Keeping him well hydrated is very important. Lots of warm mistings every day. Can you get live silkworms (Bombyx mori)? If animal stores don't sell them, I think when I was in Seoul this January, I saw fried silkworms as snacks around Dongdaemun Market. You might be able to buy live ones.
