For us, it would depend on whether this couple has decided to date each other exclusively. If they have, then we wouldn't play with him alone, and would explain why.
If they aren't exclusive, then I see no problem with continuing to swing with him.
When Mr LM and I were dating, and had been for some time, we were still seeing other people and knew that was okay. Once we committed to each other it would have been a betrayal to date others, or swing, if we had been into it back then.
Since his girlfriend has already responded positively to the idea of swinging, I think that's promising for you all to develop a new relationship together. I would leave it to the guy to develop that idea. I would think that his girlfriend would eventually ask him if he's ever swung. He then should be truthful and say yes. She may then ask if it's been with you and he can go from there. Remember, he is taking the greatest chance in losing her. I think however it turns out you three will still remain friends.
Good luck and let us know how things develop.
LM