Other forms: swains
If you want to sound old-fashioned and a little bit fancy, you can refer to your boyfriend as your swain.
Old words in English tend to accumulate meanings like old rocks accumulate barnacles, and this one's no exception. These days most folks use it as an elegant variation on male admirer, but originally it denoted a rustic or peasant, specifically a young man or boy who worked as a knight's servant. It comes from the Old Norse word sveinn, which means "boy, servant, or attendant."