I agree, you should definitely discuss this with him before it gets out of hand, but this is also something you should have discussed before it happened. I have no problem with three-somes, however, I don't think it is a good idea for a committed couple to engage in this. It just does not seem like it would be good for the relationship. Too many issues would come up. Maybe he had fun, but is now regretting it or can't get the images of you and the other guy out of his mind. Or maybe he is fine with it and you are concerned and are reading things into how he is acting. good luck.