Hmm, let's see.
He already has glasses, so I'd exaggerate those into some big absurd goggles.
I wouldn't get rid of the bright red on the striped sweater, but maybe tone down the bright white into an aged/cream color.
Maybe exaggerate the stocking cap to make it extra long, perhaps with some deer-stalker style ear flaps.
A bandolier and belt with all sorts of tools to help with hiding - maybe even some invisibility potions? Definitely some maps and other traveling/navigation accessories.
Instead of plan blue jeans/pants, how about some jodphur-style pants, tucked into either some tall boots, or military-style gaiters.
A walking stick of some sorts, with a giant topper - a globe, a compass, a sextant.