How long are you going to be in professional contact with this person? If it’s just for a few months, just keep your mouth shut and keep things professional.

If he’s a permanent addition to your professional contacts, you should seriously consider what it’s going to take for you to get over him. I have heard of cases where this sort of thing got rather out of hand. Don’t repress your feelings, feel them, welcome them, appreciate the special person that has entered your life – just don’t communicate them to him. You can enjoy an impossible crush just for what it is, sometimes.

Good luck.