I have been spotting plaid everywhere, on men and women, in the form of shirts and backbacks; and frankly, its making me mad for plaid! Flannel shirts are perfect for the last few weeks of winter, and for class! Totally casual and comfy, but ten times more stylish than a sweatshirt. My favorite is stealing one from my boyfriend and wearing it over skinny jeans and flats. They are extra roomy and have a tunic effect!
Another look to try with his shirt is transforming it into a skirt! Simply pull the sleeves in so they are hanging inside the body, situate the shirt backwards with the collar standing up at your waist and button up the back! Then with the sleeves hanging down on the inside just pull them up and tuck them into themselves – this creates pockets. Wear with a simple tee and boots & you have a fun plaid DIY project, and you boyfriend will never know!
But if your boyfriend isn't a Fashionisto like this guy you should be able to find lots of options, from your local thrift, or online, like this Vince option from Net-A-Porter!