How is cheating an accident....especially since it was ongoing? Why does he call you and tell you that out of the blue?.....because he is having problems with her now and he has come to see that the grass was not necessarily greener. He likely has also realized what he allowed to happen to his children and grandchildren. He may regret how things took place.
As was said, it is perfectly natural to miss him from time to time, and miss the family unit that you once had.
Has he said anything about wanting to get back together or perhaps he is just wanting to talk to someone because they are having problems? You are someone he has known a very long time and he may feel that he can confide in you.
You will have to decide how much involvement you want. Contact strictly to discuss kids and grandkids? Occasional civil chat? Develop a purely platonic friendship? Once you decide what you are comfortable with, let him know.
Good for you with moving forward! It gets easier and your sense of accomplishment, especially with your schooling, will be so very rewarding.