Divorce and inheritance

A married couple wish to leave all to each other substitute their only son, who is due to marry imminently. They would like a proviso that any inheritance he receives should not be part of any divorce settlement. A discretionary trust would not help in that they do want the son to benefit from the estate when they die and not wait until older, especially since there is no age when divorce is not likely. My only thought for the son is a pre nuptial agreement. Does anyone have any thoughts on this or any other practical solution?