Unless you have an existing order stating otherwise you son has no legal obligation to let you see his children. As both a parent and a Grandparent I do feel for you but legally you don't have any real options
Just curious... have you gone from NY to CA to see the kids?
Is this an outright refusal or more of an argument over travel and length of stay?
So long as you and your son are not building walls you can probably work something out... Just know that you don't get to make the rules. Play nice and you may have privileges.