Confuseddil I think your FIL has been hurtful, unkind and is behaving like a child! Unfortunately that is how some people do behave when upset or angry. As this quote from you shows *They’re close to my toddler DS and I can completely understand their upset, we’re disappointed they’ll see less of him too", you on the other hand, are behaving like an adult, making decisions about the right thing to do for all of your nuclear family and at the same time kindly considering your IL's feelings, despite the childish and unkind behaviour.
As you are sure that they won't apologise and your husband won't speak to them about it, I am not sure there is too much you can do before you go really! Maybe you can concentrate more on building ways to develop regular contact once you have moved, assuming that is what you want to do. Skype is brilliant including with toddlers. You could set up a system of sending a brief email each week, with photos of your grandson maybe? Anything that suits you that might over time show your IL's that all is not lost and that a new relationship can be built in different circumstances. It will ofcourse be up to them to engage if they choose, and if they don't then that is their choice.
I hope that your move goes well and that things work out. You can try, but only your IL's can actually solve this problem because it is their problem in the end!