A legally binding Turkish contract or a UK one? If he legally inherits part of, as is the law, then I would guess that here in Turkey the judges might overturn that contract if requested.
Thats's the main problem as far as we can understand, it doesn't seem to matter what agreements/contracts/wills etc, you have in place here, if the Turkish judges don't agree that it complies with their understanding of the law, then it is not worth the paper it's written on.
I think the problem intensifies if there are children from other relationships involved. At the end of the day the law is that no family can be disinherited so think brothers , parents, cousins etc etc.....