Headache can be because of many causes. But, it is very commonly noticed that the problem is attributed to some head injury
(even trivial) which occured before the onset of headache. In your brother's case, headache is unlikely to be related to fall for two reasons. One reason is the time gap between the fall and onset of headache (6 months) and the other reason is the investigations (which have come out normal).
Your brother should be clearly told that he may be having headache, but it is not related to head injury in anyway. A detailed evaluation by a psychiatrist
should help him understand this, which is the first step in overcoming this problem.