Personally, I feel it's much better to take a chance with some things, than spending the rest of your life wondering what could have happened if you took that chance.
Personally, I understand what it's like to have feelings for a person who is in a relationship. With that being said, I wouldn't say anything. He is in a relationship. What if the tables were flipped, and you were the girlfriend, and there was some woman professing love to your man? Wouldn't seem as justifiable, would it?
Let him know, but also respect their relationship.