Silkies sometimes have neurological problems...many try vitamins (they are predisposed to wry neck/crookneck due to their open skull structure with the large crested ones). There are often tales of silkies walking in circles, walking backwards, etc. on BYC.
Of course the brain swelling can be caused by infection, vitamin deficiencies, injury (was he recently pecked by another chicken) and so on.
It is quite possible that it is a seizure. Unfortunately I don't know how to help or give advice on that if that is the case.
If he were my chicken I'd try vitamins to see if it helped but also make sure he isn't being bullied.
For more info. on silkie skulls, you can go to browneggblueegg.com and look at their article on silkie skulls if needed. They also have an article on crookneck (wry neck) which details medical treatment for that, including doses of vitamins.
Of course I don't think it is wry neck since he doesn't have a wry neck, but am thinking the information may be useful to you.
Hope you figure something out! I'd be worried about passing on the gene to chicks, but I'm not sure how chicken breeding works. Good luck!
