What should I do?

So I fancy this guy...well, it's a little more than 'fancy', that's kinda an understatement, but I dont know how I go about telling him. Our relationship is a professional one and he doesn't seem like the kind of person to mix work with pleasure...should I tell him how I feel? Or should I just leave it and not get involved with him? I seriously really like him and I'm just at a loss over what to do :/