I will be very surprised if this behavior stops. They will go back to picking on him sooner or later, if they don't start on another bird.
Yes, I think your birds are likely molting. This is a time when it is even more likely for them to start picking. The new feathers have blood in them...
Your chickens have started to become cannibals. This is really hard to break, once they have started. I have seen this behavior several times, and really the only way to stop it is to debeak (and I'm not talking about a total debeaking) your birds. Just take about 3/8 of an inch off the top...