Never contact or respond to this man again. No further discussion needed.
There is nothing in it for anyone involved. No matter what grievances he has against his wife, he has decided to stay with her and it is up to him to deal with his feelings without any input from you. It has nothing to do with you, if he needs to talk to someone about it he should contact someone else - a male friend, a relative, a therapist, or talk to his wife. The only possible result of him having any contact with you is increased pain for his wife and increased pain for you. You clearly need to relegate this man to the past. Then look at why you went along with this.
Why do you need to be any support to him? What good can come from any further contact? He doesn't care about you, or he would not be trying to use you as a convenient easy source of attention and someone to offload to. You shouldn't have got back in contact when you got that letter, show him you don't consider his problems your business by ignoring him.
His wife is right, he is claiming to be trying to repair their marriage but is actually undermining that by contacting you. However as you say, you have no future with him. Stop allowing that, block him. They may not work it out, especially given he won't apologise, admit regret or presumably block you himself. At least it will be their business though, no role for you in this. He is going to make a mess of whatever he is involved with. Leave him to it, and try to help yourself to be someone who won't do the same.
Either block, and don't reply. Or simply reply saying that yes, you agree that you should not be in contact and have blocked him. The End.