What other sorts of things do you think Reiji would wear?
Partly due to his inferiority complex, his clothing and appearance is also an important aspect to Reiji. As we all can tell, likes to wear anything that is considered professional. He cares also a lot about his looks so it would make him appear more above than other people (or even his siblings).
One example is when, Yui actually thought of him as the eldest among the siblings when Shuu is actually the eldest. Or, his actions or form of speech sounds more mature than his immature little brothers (and shuu).
So yeah, Reiji would probably enjoy wearing suits, button down shirts (anything that you can probably wear in a professional employment). It doesn't matter if it's uncomfortable but--as long as his appearance can make an impression above others, that's all he wants.
Also, his clothes MUST be appropriate in a type of event. He won't wear a suit if he's going swimming of course or--If he has to hang out to do some shopping, he has to wear something casual (yet a little bit professional, close to a school uniform).