I agree with the above post. I think it’s important that you maintain the current relationship with your son and GC.
It seems that your son would have been quite young when your daughter died, (commiserations to you all for that), and it must have seemed to him, at a young age, that he’d lost his sister (physically), and then lost his parents emotionally. Obviously, for whatever reason, he has found it easier to”forgive” you than he has his mother.
I know that a child bereavement can cause a massive reaction in a family. We lost my baby granddaughter 18 years ago, and the shock and grief rip through a family, often leaving havoc behind.
Your wife couldn’t help how she was, and it’s good that she sought treatment. You said your behaviour was less than ideal, and it’s good that you’ve turned that around. But, if your son still feels anxious at the thought of his mother, then he is still suffering, and needs your support.
I don’t know if he’s had any sort of help to try and assist him in laying the past to rest, and to move on in a healthy emotional way, but that might be an idea for him to consider.
If he can do that, with your support, then he might reach a point where he could have an honest conversation with his mother, where she would need to acknowledge the pain she has caused him, albeit while she was emotionally fragile herself.
In the meantime, I would keep up the actual contact with him and your GC, as if you reduce it, he will feel abandoned all over again. And, to be honest, your grandchild has probably bonded with you, and wouldn’t understand why s/he didn’t see you any more.
It isn’t nice for your wife, but I would imagine, if she’s had treatment, she understands where your son is coming from, and realises this process will take time, going at HIS pace, for them ever to get any sort of relationship going again.
I hope you resolve it, it’s an awful situation for all of you.