"Either he or his mother had read somewhere that Oxford students pulled in flannels and stiff hats"

Rowing club members at Britain’s University of Oxford traditionally wore single-breasted boating blazers. These were brightly coloured or striped depending upon the college with which one was affiliated. Headwear consisted either of a boating cap, similar in style to a cricket cap and particularly favoured by coxes, or a straw boater such as that which Gerald sports. Patterned neckties or bowties completed the ensemble.

Straw boater
GNU Free Documentation LicenseStraw boater - Credit: Elf