You are truly a good person! (Sorry to hear you've had some family issues.)
I'm glad to know that he has made such wonderful progress while in your good hands.
It isn't that he
won't use his tongue, it's that he
can't use his tongue.
It happens when a cham suffers from malnutrition because there are a number of minerals and vitamins which are necessary for the cham to able to control his muscles--the tongue and its "shooting" mechanism being muscles.
You can read some more about it here:
https://www.chameleonforums.com/blogs/sandrachameleon/232-tongue-well-troubles.html
Chams do heal and recover from illnesses rather slowly.
The fact that he is still alive is a credit to you and your vet's amazement at the healing should tell you just how rarely such a degree of progress occurs.
Obviously, all these things that you are doing are healing him.
I'm a fan of Phoenix Worms' nutrition and maybe you'll want to add them to his already vastly improved diet.
You can buy Phoenix Worms in bulk at a savings if you buy from the breeder, The Phoenix Worm Store.
Hopefully there have been at least a few days where he has been able to bask outdoors.
I swear that it gives chams a psychological boost in addition to the well-documented benefit of the cham's body producing the right amount of its own
D3.
Thanks so much for continuing to keep the forum informed about his condition.