First of all; the fact that the car has always used one particular grade of fuel has no bearing on what can be used in the future. If you were talking about motor oil grades or brands; it would be different; but fuel grades do not affect future choices.
Secondly; the answer to your question depends on the engine model in this car, and on the goals your son has. There were four different engines used in the 1997 Taurus: Two of them were 3.0 liter OHV engines (one flex fuel model and one non flex fuel). Those engines are relatively insensitive to fuel choices, and would work just fine with low octane fuel. There were also two DOHC high performance engines used in this model. One was a 3.0 liter DOHC; while the other was a 3.4 liter DOHC. These engines come equipped with detonation sensors; which serve to prevent engine damage when low octane fuel is used; but this type of engine typically gets better fuel economy, performs better, runs cooler, and lasts longer when higher octane fuel is used. The savings in fuel usage with this engine type often offset the higher cost of mid grade fuel.
You can find the engine model used in this car by reading the information on the emission information label; which is found in the engine compartment; typically fastened to the underside of the hood. If there are the letters "OHV" in the engine description; this car can use regular fuel. If the letters "DOHC" are on the label; it is a high performance engine, and will run better on mid grade.