What you should do depends on what positions you both hold at your jobs and what kinds of signals he's given you up until now.
First, I want to say it's okay to tell a person you're interested in them even if you're not sure if the feeling is mutual, as long as you're prepared for how you'll react if they don't like you back. But when it comes to a workplace crush, it could be super awkward! Are you prepared for that and are you okay with working alongside him even if he turns you down? If not, I wouldn't say anything.
I also wouldn't say anything if you are his boss or he is yours since that could create an imbalance in the hierarchy. If one of you is interested in the other and has authority over the other it could become problematic on a professional level. If you're both employed there equally and you're alright with being rejected - go for it! Ask him out to do something fun!