I was wondering that too... Then I thought no, it was probably a school uniform... but his colleagues didn't wear uniforms in the chapter (37) where Hayate first heard Ayumu's confession.
Then it occurred to me that the first page said "April", it's the beginning of the school year in Japan, so they were probably both going to the opening ceremony of the school new year, and Hayate just put on a suit...
... or maybe Ayumu just like to dream of "Hayate The Butler".