I only recall one incident of him having a headache - when he arrives in London, after a all night drive down from Manchester, where he had been drinking. It's only natural for him to have a sore head.
As regards other pointers towards mental illnesses; I don't see them. Clearly, he has had an unstable upbringing, which would account for him slipping through the net of society, despite being highly intelligent. He is frustrated and angry.

Classic ENTPism that Johnny displays: Thirst for knowledge, unkempt appearance, directness to the point of being abrupt, vocally outspoken, enjoys getting a rise out of people......loads more.

I am not saying all ENTPs display these types of behaviours but when you look at Johnny and observe other ENTPs, one has to agree that he has a manner similar to Bugs Bunny or Groucho Marx (classic ENTPs characters).