Is he in the quarantine tank because your putting him with others?
If he is going into his own tank then get him in and feed with the bowl method. Keep the basics in mind like quiet traffic area and no night lighting.
If he is going into a tank with other frogs:
If he is a juvi, you may find his removal from a group has left him stressed. Add in the trauma of his last probably 10 or so days of packed, travels, sold, and now in a totally different place and you have a pretty depressed/stressed frog.
I used some lucky bamboo placed into some stone in a larger cup and found the juvis I had would use the cup to soak and hide. If you have some leaf liter, toss some around the bottom also and mist the liter to keep it from drying up. They like to hide under it and the moisture from the leaves will be good for the shed.
He might just need more time to relax.