So I wondered if this was a personal choice on his part or symptomatic of a larger Iranian political trend. I asked a very savvy Iranian source about it, and here is his fascinating analysis:
This issue goes back to the immediate aftermath of the 1979 Revolution. Before the revolution, all public figures in Iran and all officials wore ties, both domestically and when on visits abroad. Shortly after the revolution however, the tie itself began being associated with "Western imperialism", especially after
Ayatollah Khomeini branded a large group of intellectuals (who were less religiously zealous than he would have liked) as "tie-wearing cronies of the West" and essentially branded anyone wearing a tie as being Western influenced. As such, no Iranian official since that time wears a tie, whether in Iran or when on official trips abroad. In fact, for many years after the revolution, the site of a regular person wearing a tie in Iran was so rare that heads would turn on the street and funny comments would be made if someone wore a tie outside. Many people still wore them to parties and weddings and things, but it was very "taboo" during the 1980s.
Gradually, as Khomeini's legacy became a bit less overbearing, regular people stopped caring and the rhetorical plays on people who wear ties as "imperialist cronies" were no longer made, meaning that at least ordinary people now wore ties on a regular basis. I myself for example, always wore tie at work in Tehran, as did many of my colleagues. I would actually make a point of wearing a tie outside as much as possible, to do my bit in making sure that people got used to seeing other people in ties.
On the official side however, wearing a tie is still a no-no and it would be unthinkable for Ahmadinejad, who claims to be one of the "true disciples of Khomeini" to sport a neck-tie under any circumstances.
Interestingly however, many of the children of regime officials wear ties in addition to having outlandish dress and hairstyles in general (which are often criticised by the hardliner newspapers), without their parents having any real control over it.