What should I do next?

Ask him how the party went the next time you see him. Then suggest doing something together after that. Keep it casual and don't stress about it too much, he likes you!

Delta Boy said it well. Show him that you sincerely feel bad for missing his birthday and that you would love to get together some other time so you are able celebrate his birthday

Well he obvious likes you to ask you out for his birthday, but it does suck you can't go. So you need to turn a bad into a good. Next time you see him, say you feel really bad that you are going to miss his birthday and want to make it up to him. Say that you two should go out somewhere to make up for it; movie, dinner, etc. That way he not only sees you care, but it is just you two so the attention is much better!

If you like him and he showed initiative to spend time with you, definitely return the interest. Do you know how many of us girls wish we had that kind of door half way open for us. :) if that is what you want then ask him, if he is interested in getting together another time, ask him for a drink or coffee, movie whatever. What have you got to lose?. :)

I think he likes you. You should try and do something with him. Go out somewhere. It doesn't have to be a date but you could plan it and say it's "pay back" for the unattended birthday. He would like that and it would reassure him that you were interested.

I totally agree with the responses. This is a perfect opportunity to say 'hey I'm so bummed I can't go out with you to celebrate your birthday, so let me take you out to celebrate another time. " Just be careful about what you say at work. People gossip and can create drama. Keep your private life private at the workplace. Take it from my experience. I worked with this guy, got to know him, developed a crush, people we worked with said we made a great couple and it got uncomfortable.