What a tough situation. Maybe contact her and let her know that it would be nice for your ds to know his grandma and see what she says? At least give her a chance to respond - she may be feeling terrible after finding out her son is your ds's daddy, but after ignoring you for some time, she may be feeling too ashamed to initiate anything (and may be secretly hoping you will contact her)

If it doesn't work out, it's okay, I mean it's sad, but more so for you...your ds won't know any different and he has a wonderful loving mummy!

GL