2003 Chevy Malibu Coolant Level keeps dropping

If its the thermostat it will cause it to overheat. If there's no actual leakage I suspect that coolant is being burn in the combustion chamber due to a blown headgasket you might want the tests below done.

Have it block and pressure tested-do the block first to pinpoint a combustion leaking into the cooling system or a gas analyzer to sniff for hydrocarbons at the radiator fill neck.

Pressure test: do not do a pressure test if there's leakage at the headgasket this might cause coolant into the cylinders and lock it up or bend a connecting rod if cranked thereafter. The headgasket should be repaired before doing the pressure test.

Have your son check about 100 miles to see where the coolant is going, check for physical leaks, if none or noted then it is probably internal. Post back.