call up during the day from work when he's probably at work and see if there is a name on the answering machine.
*or*
simply call the number and say to the guy, listen, I met you (whenever) and we had a great time, and you gave me your number. Me, being so silly (haha *laughs*) forgot your name, but I do remember having such an amazing time w/ you. So, you wanna go out?
If the guy remembers you and likes you, I don't think he's really gonna mind that you forgot his name.
I know I wouldn't give two shits if a girl forgot my name after meeting me at a bar...