Is he in a 'real' crib? If so I would move him to a playpen as they have soft sides and his head banging will have less of an impact (on him and you). Make sure the playpen is a legitimate napping playpen where the bottom mattress straps down as the head banging will likely flip the mat up and he can get tangled.

You would have to figure out why his is doing it. If it is for attention and to get what he wants then you need to find a way to ignore him and not give in...BUT obviously this is hard when he is banging his head and has the potential to harm himself. The playpen may work...if it helps then I would have one set up just outside your main playspace and when this behavior starts remove him with no words or attention and leave him in the playpen where he is safe.

Keep note of other atypical behaviors as children that head bang may have other 'issues' that need addressing but again, at this age it is so hard to tell if this is just a severe tantrum behavior (which would be classified as normal) or if it is a sign of a deeper problem.

Hopefully the parents help you out here!!