Oversize sweaters, jumpers, shirts, jackets… There is something special in them. Not because these should obligatory be a part of your boyfriend’s garderobe, no, such a piece can be totally yours. But you can try wearing his apparel too just to feel the vibe.
As for me, I really like wearing extra size dresses and other pieces of apparel because these provide me with a feeling of “being inside the dress”. This has not only this effect, but also makes you look smaller (compared to the size of your wear). Once when I was a teenager, I wore my father’s checkered shirt with my mum’s velour jacket to the party. It was a period of raves where the rule was like the odder the better, the older the fancier. But this is a bit another story.
Extra size jumpers and sweaters go well with skinny jeans. Bold shoes like long boots should balance the oversize, though, not to make you appear proportionless. The layers are welcome too. Try putting on some thin shirt that is a bit longer than your sweater, it will add extra volume and make the contrast between your skinny trousers and volume upper part of a dress a bit more gradual. A small over the shoulder bag could be a nice extra and a final touch.