Oh Jeez, that's just NOT ON.

Here comes a rant

Naming your child the name you have given him and registering him as such is the only thing to do. It is not drastic action. It is your son's right to his true name - the one given by his parents.
IMHO if you don't, you may as well tatto your MIL's name across your own forehead, what could be next? Take a stand, claim your place as his mother and your healing process can begin. One of the best things about giving birth is how protective we get of our young - tap into that strength, and wear it with pride mumma.

rant over..

Can you take some strength from your own parents on this? I'm sure they were as shocked as you at this behaviour.
Keep us "posted", we are all here for you.