You can try to spot feed it a little. Maybe a couple small pieces on the tentacles.  Don't put the food in the mouth.  If he's hungry, he'll walk the food up to the mouth.  
As nano said, if it starts to decline don't get rid of it.  I had a plate that was 99.9% skeleton, with the smallest strip of flesh still left.  And after moving it to another tank, it's now back to 85% of what it was originally, with two small baby plates growing off it.