Let him know, but also respect their relationship.
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?
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.