Hun, it's my guess that: 1) She probably knows about the DNA test outcome (so is aware she has a GS); and 2) She can use a phonebook and find you herself if she wanted to.

Assuming those two things are true, I think you can let yourself off the hook. Should you hear from her now or in the future, then you can decide how much involvement she should have in her GS's life. I wouldn't contact her, it's not like you've moved to another country and are in hiding. Let her find you and make the effort if she's interested. Save yourself the heartache of probably being rejected or having to work to encourage her involvement. Your DS would rather one happy Mummy than an unhappy Mummy and a clearly manipulative Grandma.