asking him on a date

Soooo I'm a Starbucks barista. There's this customer who has been coming in daily the last few weeks and we're flirty enough for other to notice. He found me on Facebook, and we've started talking more when he comes in. I want to go to like dinner or something with him but I don't know how to ask. I feel like writing my number on his cup is so cliche but I've considered it😂 anyone else have ideas or advice. I'm not ballsy enough to just say hey let's go to dinner