If finding the other arguments is a complex operation (i.e., cannot be reduced to a single expression) you can add a static method that do that for you and refer to it in the super call, something like:
Class Son extends Base {
private static boolean getMyBoolean(int num) {
return num > 17; //or any complex algorithm you need.
}
public Son (int num) {
super(num, getMyBoolean(num));
}
...
}
Otherwise, if the missing arguments can be calculated using a simple expression (as in the concrete example you give), just write:
Class Son extends Base {
public Son (int num) {
super(num, num > 17);
}
...
}