Eating doesn't matter as much before vet as hydration does. Definitely focus on getting him stable with hydration before hand. He will need showers the first day you get him from the looks of him and I wouldn't go less than 30 minutes each time if you can help it. If you can get longer do so. Do a shower at least once a day. Twice a day if he will let you. Do you know his care at all from his home right now? Like is there any possibility something has cause an RI? If not he may need to get wormed especially since he is likely wild caught and has a parasite load that has grown too large for him to control. Ask the current carer, that you are getting him from, what he has done and anything he may have noticed. Usually warm showers are the best way to get furcifers of any type to drink. Make sure it stays warm while he is in it. I set a time to check every 5-8 minutes. Vet should be done once he looks a little more plump and is drinking rather well. If he starts eating while under critical care that is great, but don't try for the next two days I would think. Even if he starts to spiral down... in my opinion taking him to the vet without getting him somewhat hydrated before hand could be a death sentence because of the amount of stress. Though there are definitely situations and things a vet could do that would help. But I have had plenty rescues I have taken in who were critical that they gave fluids to and that was it and they died a could minutes later just from the stress. I have since started keeping saline in the house. I can usually give a small amount to the chameleons by myself, as is with my other animals, except the birds, won't touch the birds. Usually doing it at home, could very well likely push them over the edge as well, but I also have had greater success with it. I don't recommend this for someone who has just started rescue care. I think he will do rather well after several showers through out the first week, and if you have any soft bodied feeders (hornworms are the best but silkworms are great too). These can help with hydration once he's towards the point of eating. Good Luck hun, rescuing can be a thankless task in some ways. You can spend hundred to thousands of dollars on an animal who gets to a healthy life and then only dies a year or two later from the import process or something else that happened in previous care. But if you are willing to try and definitely willing to spend money to at least get him checked out and helped where you can, you can do a lot to change this little guys life around. Kudos to the person who is giving him to you because he obviously just needs someone a bit more experienced. If he doesn't show interest in eating or shows issues with having enough energy and possible deficiencies I have give a liquid diet (which helps a lot with hydration as well) that I end up blending bugs and putting vitamins in, with a few healthy veggies and fruits. If you want to know my exact method shoot me a pm. It's a bit different than some of the other bug juices I have come across.
I would say if you can get a vet visit either next week or the week after that would be great. Get it a little bit out. Work on getting him hydrated for the next week until then. And then maybe start trying to get him to eat. For the liquid diet it would be best if you have 1 cc oral syringes. So you might want to either order some online or get some if you know somewhere local just in case this becomes a necessity and you have it on hand! You have some odds stacked against you two, but if any animal can get through a hard time it's chameleons. It can take them months to a year to show good improvement. But they can pull out of situations that is just amazing.
