Puffers teeth grow continually and they need hard snacks to keep their teeth filed back. For my puffer, I have live rock in the tank that he picks at, I also give him occaisional cockles/baby neck clams. Some people toss in a shrimp from the grocery store, still in the shell.
If the teeth are allowed to get to long, they will eventually starve to death because they can't eat or their mouth gets stuck.
I have read articles about some puffers needing emergency dental work to file down their teeth so they can recover.
I don't think a QT is what he needs right now. Post a picture please - if you can get a picture of his teeth and mouth, that would help - we can take a look.
Below is an article about puffer teeth, helping the fish keep them short and how to do emergency dental work for your fish. Good luck.
http://www.wetwebmedia.com/ca/volume...ry/puffer2.htm