1. If he's interested in you, too.
The premise of this advice is that you find your FWB interested in you and invest more time, energy and money in you than a FWB should. For example, when you have sex, he always tells you to rest in his room for a while, or drive you home late because he is worried about your safety; when you tell the hookup finder about your troubles in the day, he will find ways to amuse you or help you find solutions, etc. He devoted his precious time and energy to you. As you know, a NSA relationship should be centered at intimacy. Only that he really likes you can explain his behavior. If you are also interested in him, then you can take action to show him that he is more than just a friend to you.
2. Find many reasons to be with him
Try to ask him out on dates, such as eating or watching movies, and all the other things that ordinary couples do, instead of always having sex at home or in bed. If he's willing to accept your invitation, have brunch at a local cafe, or attend a concert in the park, that's a good sign. If he is not only willing to stay in bed with you, but also willing to spend time with you outside, it shows that you have a great opportunity to make your quick flirt relationship more serious. If you have common friends, you can create more chances to get along with them; if not, you can find or create more chances. During these times, you can get to know each other better than in bed.
3. Don't focus all your attention on him.
Although this may sound counter-intuitive, because you want to establish a loyal and serious relationship with him, from the beginning, you make him feel that he is your only one, which is not good for you. When you are alone with him, he can be sure that you will not leave him, and when he wants, he can always find you. This will lead to the result that he will not cherish and value your existence. So you need to make sure that the FWB around you is not just him, but you need to shift some of your attention to other people. This will give him a sense of crisis and make him crave your attention and love.