It is simply a mistake to use constructions such as “utility gain for the gifted son from living in the suburbs would be larger than the utility gain for the disabled son from living in the city”. Utility is first-person relative to the decision-maker. Your assessment of the utility of each outcome is dependent on your subjective preferences relative to the options. You, the decision-maker, do not, and cannot, know the subjective utility values that your two sons would assess to their own outcomes.

Subjective utilities are not fungible, you cannot ask Son A how utility he expects and then compare that number with the answer Son B gives you.

Thus, if you prefer A to B, yet you somehow wrote down that the utility of B is higher than A, you just made a mistake somewhere. Utility is just a scalar valued encoding of subjective personal preference. If you are using it in some other way, e.g. pretending that you are accurately measuring the subjective utility of your sons (rather than your own subjective preferences) than you are going to get weird and usually useless answers.

Some people do use utility in such a way that they assign numbers to other people’s well-being, but doing this always leads to unresolveable paradoxes such as the Repugnant Conclusion, because it’s just not how people think and decide, nor should it be.