A couple of thoughts that no one else seems to have thought of, and which could, of course, be wrong. Perhaps, just perhaps, the reason that your son brought you into this was a 'cry for help'. Living this double life must be exhausting and he wants it over but he doesn't know how to do it. He knows he is lying to his wife. He knows he is lying to his other girlfriend. Up to now he has been lying to his mum. But at least the truth is out there now and not a burden he carries alone. Hopefully he realises that telling you the truth has not meant you completely disown him, but still love him even while you hate the choices he has made.
I only see one way out of this tremendous mess and that is for him to have the courage to confess to everyone. Now, that may mean the end of both relationships. But I can't see how either lady, if they discover the truth of their own accord, would want to stay with him. If he is a normal man, I am sure he would rather have a whole relationship based on truth than a half-relationship based on lies. Truth will mean that he can win back a little of the respect that he has lost from you, and will lose from his wife and girlfriend. Their worlds will no doubt be shattered, and so will his, but so much better to rebuild aged 26 than aged 31. And maybe he will regain some self-respect as well, since that is taking a battering.
If you tell DiL, then you deny your son the chance to show the courage that may be his only redemption. There are no best-case scenarios here. From what you wrote (and trying to avoid the torrent of other comments) it appears that your son would rather be with the other woman in which case he should not stay with his wife out of sympathy for her. However, unless he confesses to OW that he is married, how could he ever bring her home to you - given what you know about him? But this is the risk he is going to have to take. He has to confess to both. My hunch is that it will be the end of the marriage. I hope he would still want to see and raise his son. And for the other lady, she will have to decide whether he is worth taking a risk on. (And there is a very, very slim chance that the reaction of his DiL to his truth-telling may just turn him back to her - but the chance is very slim)
Meanwhile, OP, for you. You raised your son to be the best man he could be and to fulfil his potential. At 26, it is not too late. He feels he is a fully-grown man, but he still has much learning to do, and there is much that you can still input into him. While your deep, deep desire is to stay in a loving, warm relationship with everyone, this is out of your hands. It will depend on your DiL and on the OW. The only one who will always come back to you is your son. So you need to confront him with truth and challenge him, and love him. Cry with him out of pity as much as anger. But I highly recommend that you prioritise this relationship as it is the only one, from a distance, that seems like it is certain to continue.