Courting someone who is married is no different than courting someone who is unattached. Just keep up whatever it is you are doing. The reason why he’s telling your mutual friends that he would be with you if he could has a lot to do with the fact that you are sending him signals that you are interested, even if they are subtle. Guys are usually not as stupid as they look, they can tell when a girl “really, really” likes them.
He may be thinking that he’s ready to move on from his wife. He may thinking that he would rather be with you. This happens. Sometimes marriages don’t work out. Sometimes the wrong people are together, and it takes the right combination to make that apparent.
One thing you can do, however, is work yourself into a situation where the two of you are alone for a lengthy period. The best scenario would include drinks. If he is leaning towards a relationship with you, it will manifest itself here.
By the way, should things work out with you guys be prepared for the inevitable cries of “home wrecker!” Ignore this. Life is too short.