I've had a couple of chameleons with no tongue.
Hopefully the vet took it off leaving just enough skin to cover the hyoid bone....if not it makes them uncomfortable and makes the chameleon struggle with the loss because the extra skin seems to affect the way the tongue feels in their mouth and they keep adjusting the bone in their mouths.
It's not usually hard to train them to eat like other lizards (by walking up to the bug or chasing after it). To get them to eat on their own again...you just have to hold the insect near their mouth (after first showing it to the chameleon so it realizes you have food)....move the insect a little further away each time you feed it and it will soon figure it out.
At first it may be uncomfortable with the loss of the tongue in that it will keep trying to shoot out the tongue but it will soon realize it has no tongue.
Good luck with the training!
The fish thing is strange since they don't eat fish...although feeding him softer bodied insects for a few days would help I think.