Dp's name is on both dses birth certs and both registrars said that he'd still have no parental rights over them.
Harman, I hate to say this, but as much as your MIL is a cow, she is still your child's grandmother and will probably be upset that she can't see him. Just because she's a witch to you doesn't mean that she's going to be horrible to him, and it sounds like you only won't let her see him out of revenge.
Imagine how you would feel if in 20 years time your son had a child with a woman who didn't like you, and wouldn't let you see your own grandchild.