Thats great you took him to the vet. You're already on a good start!
Since he's not eating, most likely it's because he's in a new environment. It'll normally take a chameleon a week or two to feel comfortable enough to be normal again.
But since he has a R.I. you're gunna have to stress him out a bit. But it has to be done. Force feeding is pretty tricky. If your cham gets mad and tries to snap at you when you open your cage to grab him, try and hold a cricket with the meds dropped on it so he can bite it. Usually they'll end up chomping it down and eating it.
Another thing you can do is hold him in one hand, use the other to get his mouth open and have your girlfriend toss a med coated cricket in his mouth (This is what I had to do). Getting their mouth open is the tricky part. Sometimes chams will open their mouths if you gently press the sides of their mouth. If that doesn't work, I used a credit card. Be sure you sand down the edges so it's not sharp. Use it to open their mouths and have your girlfriend stick the cricket in so he can bite down. Since you have to force feed a med, you might as well give him something to eat. Some other options for feeders to use are silkworms, hornworms, superworms, dubia roaches, butterworms, calcium worms, and the occasional waxworm.
You can also buy a small syringe and try to get it deep inside your chams mouth, but I'd only recommend it to experienced cham owners. I myself don't like doing this.
Regardless your cham will be stressed, but they'll get over it.
Like I said, it'll take a good solid week or 2 for your cham to get used to his new home. But you're def in a great start and your cham will be back to normal in no time.
Check out this thread for additional help//info:
https://www.chameleonforums.com/how-best-force-feed-32372/