Many of us here were given the wrong care instructions when we first got chams, so we do understand.
You care and you want to learn and do what's best for your cham.
Those are the qualities that make for a good chamkeeper.
Kinyonga is right on the lighting, calcium dusting and the fact that your cham may have an abscess, which needs veterinary treatment as soon as possible.
It is important that you get him to a vet who regularly works with chameleons, or at least with reptiles.
In Florida (I know it's a very large state), there are some excellent vets and these 3 come to mind.
If none of them is in your area and you'd like a recommendation, there are a number of forum members in Florida who may be able to help.
You can also have a look at this page , click the 2 links there
https://www.chameleonforums.com/care/healthx/vet/
The first, Dr Ivan Alfonso became a vet because of his interest in chameleons. He makes house calls only, if I'm not mistaken
http://www.kirkmanareavet.com/
Next, is Dr Oz on the chameleon forums who also raises chams, though I don't know if he still maintains an active veterinary practice
This is his contact page for his cham sales, which may be the only or best way to reach him
http://www.dros-tiedyedchameleons.com/contact.html This doc specializes in reptiles and writes extensively about their medical treatment, Dr Doug Mader
http://www.marathonvet.com/about-us/veterinarians/dr-doug-mader
Hopefully, you'll get an appointment without delay.
In addition to what Kinyonga pointed out, there are some other things in your cham care that need to be changed in order for your cham to regain his health and maintain his health.
The first is his diet. While crickets and mealworms are fine, the more variety, the better. Different feeders provide different amounts of various nutrients.
A feeder fed a poor diet will provide poor nutrition.
Feeder fed a nutritious diet will offer a nutrient-packed diet.
More info written by the moderators is here
https://www.chameleonforums.com/care/food/
Secondly and very importantly is his supplement dustings on feeders.
While you're right in that oversupplementing is possible, very often we see the ill effects of undersupplementing.
His loss of the use of his tongue is most likely the result of a vitamin or mineral deficiency. More on that here
https://www.chameleonforums.com/blogs/sandrachameleon/232-tongue-troubles.html
You already now know that he should receive calcium without D3 on most feeders.
In addition to that 2x each month he should have a dusting with a multivitamin such as Reptivite
Also 2x a month, he should receive calcium with D3 on his feeders
Most people don't realize that a UVB bulb needs to be replaced every 6 months.
Even though the bulbs continue to light up, they stop emitting enough UVB after 6 months on the average.
18"x18"x26" is very cramped for an adult male Veiled cham
24"x24"x48" is really the smallest size enclosure he should be in
LLLReptile, DIY Cages and other forum sponsors sell affordable screened cages Sponsors' links here
https://www.chameleonforums.com/index.php?page=sponsors
There's a wonderful Veiled cham caresheet here with any info that I may have missed
https://www.chameleonforums.com/care/caresheets/veiled/
I hope that he is back to good health very soon and that you will update us when your vet tells you what is wrong.
The info you share will help others.