According to Role Reboot magazine, men can wear anything they choose to wear. Generally, men and women are not the same size or shape, but if an appropriate size slip can be found, then a slip designed for a woman could fit a man.
Gender roles and norms are determined by society as a whole, and people who want to flip the accepted gender roles are free to do so. There are many online groups and forums for men who choose to wear slips or other clothing normally designated for women. There are even companies that specialize in larger-sized slips and camisoles for men.