Give him a tender, let him sign it, and then stick him on the end of the bench where he doesn't get any playing time. Let's see what his market value looks like next year, when we still retain his restricted rights.
There's nothing wrong with a rookie playing hardball. If he wants to jeopardize his NBA career rather than go into a good situation, so be it. However, the Celtics can put the screws to him, and I don't have any issue with it.