I think I would take a slightly different slant on this.
I would say that young one has come home very upset/marked/bruised as he has been struck/pushed/shouted at by an unknown child .Poor little thing so traumatised, he won't say who it is. What can they do to help? Please could they keep a special eye on him, see who he plays with, see if they could possibly work out what the problem is, using their expertise with young children etc etc.
Did this with DD2, worked a treat. They know (probably) what is going on. It kind of gives them a final warning and, if nothing else they will make sure that they keep the bully away from your kid.
By the way, if he ever comes home with marks on him, ask to see where the "accident" was recorded in their incident book. Soon sharpens up a few minds too!
Good luck, i would echo what others say, try to avoid taking him out altogher. He may well go to the same school as this child/other similar kids and it might just send the wrong message. However, could you reduce the number of days for a while?