What Alenushka said. The question is, is the incidence of new-onset psychosis cases in people who have just received the YF vax higher than in the general population? How many people have received the YF vax relative to the number who have had such an episode? And there wouldn't be a trigger event for the general population, so you would just have to survey some random sample of people (and it would have to be a really big sample to get enough statistical power), follow them for a month or so, and see how many have a psychotic break. Then comparing those numbers would help show whether it might be related to the vaccine or is random.
According to Wikipedia, over 500 million doses of the vaccine have been given. Even if the number of cases of psychosis has been underreported by 99%, that still means 1000/500,000,000=0.0002% of people have a psychotic episode after the vaccine, or 2 out of 1 million. I'd be really surprised if that was higher than the background rate in the general population.
His psychosis started right after the vaccine, but we can't know for sure if it was or was not a coincidence. Even assuming the vaccine did trigger his psychosis, I don't think that's any reason for anybody else to avoid the vaccine, because 2 cases out of a million vaccines is ... really not that many. YF isn't a routine vaccine. It is only given to people who are traveling to an affected area, and the chance of severe sequelae from the yellow fever disease is a lot higher than 0.0002%.
On the other hand, it sure would be nice to have information on the overall rate for psychosis triggered by various medications and whether it's more common in those with some kind of physiologic predisposition--and what such a predisposition would consist of, for that matter. There is not a lot of information out there about this and I do think it's a field that should be studied more.