Write a letter to the airline commending the service from the FA. Include the details that you know such as flight #, date, first name, etc. make sure that you include your contact information (telephone #, e-mail address, etc.).

The worst that can happen is the FA gets a gold star in his record. And maybe, although it is a long shot, he will get a copy of the letter and contact you. In either case, you have respected his privacy, and if you really like a guy, you will respect him.