It appears to me, that "someone" is helping his feathers fall out. I have a helper - she likes bottoms - when she plucks a feather out of her life partners behind - that is pretty much how it looks. I have tried every remedy known and made up and still it continues. I would search for "feather picking" on here and start the workng your way through the list of "fixes" until you get to the one that seems to work. If you have some, try sticking some vicks vapo rub on the fellas head - now that it's read it will become even more attractive. THere are several other things you can buy to put on him that taste bad and keep the offenders at bay - but you do have to apply them regularly. Then you will just need to watch and see if you can catch the offender. The tophat birds are very attractive to other curious chickens!
Good Luck!
Jenny
ETA - WELCOME! Sorry you got missed!