He might be feeling vulnerable because of his head. Vaulted skulls are a deformity that compromises the health of the bird, I don't know why anybody would purposefully breed for that (and yet people do). The bulge of the vault has holes in it (in the skull) and the brain is unprotected in those areas. A good peck to the head in just the right spot can kill a vaulted chicken. Maybe that's why he freaks out if another chicken tries to peck him. Get him a friend if you can, but make sure the friend is lower ranking than him, to avoid him being on the receiving end of pecks - so definitely a female, and if possible smaller or younger to ensure she's below him in the hierarchy and won't be pecking his head.
Here's what some vaulted skulls look like:
View attachment 2883319
And here's a normal skull - the bone covers the top entirely, protecting the brain as it should:
View attachment 2883320
A lot of people don't realize what the vault really is, or the ethics of it. Now that you know, please consider not supporting this barbarity. There are normal-headed silkies which you can get instead, with crests and everything, and they look just as cute (but are normal and healthy).