Look, I don't agree with your FIL's behaviour, but I do understand where they are coming from.
As grandparents, our little grandchildren are very precious to us - it's like a wonderful gift that is given to us to cherish and love. You say they are close to your little boy - no doubt they love him dearly. And all they can see is that you have decided to put your job before the relationship they have with your child. You are taking him away from them, and they are upset. Obviously that upset has been expressed badly, but I suspect that this is what they are feeling.
I know how they feel - my daughter and her husband are moving away next month and taking my precious grandchildren far away. Our closeness will never be the same again and I'm heartbroken. I'm not acting out like your FIL is, but inside I'm shattered. Maybe he is too, thinking of how much they are going to miss the little boy.
My suggestion would be - cut them some slack. Instead of waiting for an apology, try having a good talk to both your inlaws about the move. Show them some sympathy . It won't hurt you to do that, and it might help to heal the situation. Talk about how you can all Skype each other, send little messages and pictures to your son, and visit sometimes.
As grandparents we are totally powerless when it comes to our grandchildren - they are not ours but we love and cherish them. If you can see how your in-laws feel, it may go some way to smoothing this over.