I agree with TwoCrows that it could be juvenile moult or feather pecking. They sure can be pesky at doing it when your not looking!! I had some hens strip a polish boys head in the space of a couple hours while I was out. It's a good sign you have feathers coming back in but this may cause more pecking when they come through more. Do you have him in a mixed flock? Even other polish can be a bit of a pain when it comes to having a go at each other.
Your main priority now is to make sure they don't draw any blood if it is feather pecking. Once they see blood it becomes a vicious circle and the blood tends to send them into a frenzy, you could tie or band his remaining crest up so they can't see the redness. Or put some blue kote on him. Not too tight but just enough to keep it up. Also I would recommend some anti peck spray on him. Alternatively you could seperate him untill it comes back fully just incase.
I don't think it is anything serious right now but just one of those things that can happen with polands. For all their looks and prettiness they do take a little more time to care for and can throw you problems with their fancy crests.
Wishing you the very best of luck with him, I hope you will be able to post some pics when he has his beautiful crest fully back
ETA ~ Here is the link for a popular polish thread ~http://www.backyardchickens.com/t/355316/polish-thread/0_30
Please do drop by, lots of pics of polish with their crests tied up and some others that have the same problem as your boy.