No contact from son after marriage

I was estranged from my son shortly after he was married. Our relationship had been very strained from the time he left home at 18, and when he married at age 28, we were not very close. I tried to be supportive but I was not included in any wedding plans. After the wedding, it didn't take much to lose what little bond was left. He had moved to a different city many years previously and rarely visited where I lived. After his marriage, he rarely called/spoke to me.

A few years later, I sent him a letter stating I was hurt by his indifference and saying that I would rather have no contact with him than the little bit he bestowed on me once maybe twice a year. I said even prisoners call their mothers on Mother's Day. He did not respond. Even when I was hospitalized and his sister told him so, he did not contact me. This went on for 7 years. It has only been in the past year, after his son was born (my only biological grandchild) that he and his wife have called and even FaceTimed us. I am grateful that he has included me and his stepfather in the baby's life. We aren't sure what has changed his feelings but we hope that it is permanent. I couldn't stand to be cut out of his life again. I am going to speak to him directly when we go to visit our grandson in the spring, and apologize for whatever I did that made him pull away from me to start with. I guess we can only go forward from here.