Sorry, that is not good news. Best case scenario he is very stressed, worse case he is dying.
How did you acclimate him? They are very sensative to changes is water condition. Also, what are your water parameter? they are very sensative to poor water quality.
Not much you can do for him, keep good water conditions in the system and if you have a hospital tank, you may want to move him there and try to feed him small pieces of meat.
What kind of Urchin is it and what is you water temp? Some are cold water and some are warm water animals.