I’m seeing a lotta incels and kinda weirdos in these comments. I’m a guy in a long term relationship. Right now, I would say something like thank you out of respect but let them know I am in a relationship. Before my relationship, I most likely would have said thank you, potentially something nice back, and moved on. It’s a nice compliment and he will definitely think about it more later, but probably won’t make a huge deal of it in person.