You might say that "nothing has happened" but I say something already has and is continuing to happen. Some people may argue that it is not technically cheating, but you are headed down a slippery slope.
Right now you might have a little crush on him and you like the way he makes you feel, but that's just a signal that you need to work on your marriage more. If you don't want someone to get hurt, stop this business NOW.
My husband's ex-wife got was in a similar situation when they were still married, only it was with a man much older than her in a senior position in the company they worked in. He would take her out to lunch and give her spending money. She would confide in him how unhappy she was in her marriage. DH suspected something was up but she always denied it and insisted that he was not someone she would ever be attracted to anyway. However, it was 6 months later that her and her hubby (now my hubby) were separated and a year after that, all of a sudden her relationship with this other man was "out in the open". She still maintains that she never cheated. Well, maybe, maybe not. Maybe nothing physical happened. But their behavior and actions were not exactly above board either. She was cheating emotionally, whether she realized it or not. If you are having marriage troubles and you are confiding in someone of the opposite sex about it, rather than talking to your spouse, that's a huge red flag in my book.
What would be the consequences at work if you got into a relationship with this man, married or not?
How would you feel if another woman behaved the way you are behaving toward your husband? Or if your husband was acting toward a woman he worked with the way this man is acting toward you? If you cheated on your husband, and you were the reason this man cheated on his wife, would you be able to live with yourself?
Do you really want to get involved with a man that would cheat on his wife?
Both you and your husband took vows to be faithful to each other, in both good times and bad. If you feel that things have deteriorated in your marriage to the point that it is not worth saving, then do the honorable thing, and get divorced before you start a relationship with someone else. Otherwise, you need to concentrate on your marriage and get whatever help is needed - counseling, etc.
You need to get your head out of Fantasyland and realize that the grass is not always greener on the other side.