Give them unique hobbies or interests. For example, if he's into rare book collecting or extreme sports. It makes them different from the typical male character. Also, a unique accent or speech pattern can set them apart.
One way to make male characters in romance novels stand out is by their past experiences. If he has traveled the world and has a lot of interesting stories to tell, it adds to his allure. His body language can also be a factor. A certain way of walking or gesturing can make him memorable. Moreover, the way he treats the female lead in small, everyday ways, like remembering her favorite coffee order or always opening the door for her, can make him stand out as a gentlemanly and unique character.
To make male characters in romance novels stand out, consider their values. If he values loyalty above all else, it will show in his actions throughout the story. Another way is through his style of dress. Maybe he has a very distinct fashion sense that catches the eye. And his reactions in unexpected situations can be a great differentiator. For instance, in a crisis, he might be the calm one while others panic, or vice versa.