If he is so unhappy, why is he still a married man? I am sure he has given you a hundred excuses, or will if you ask him. But at the end of the day thousands of unhappily married men leave their wives and children.........regardless of the consequences. Those who do not generally are being dishonest about how "Unhappy" they are in the first place.
As far as why you should not, here are a few very good reasons:
The fact is at the end of the day if you don't hold out enough respect for yourself then it is only you who will get hurt in all this. By giving him the OK, you have just told him that you do not value yourself. You are willing to sit back and allow him to put his bad marriage ahead of you while you settle for what ever is left.