My son has a 1998 Ford Contour and after his last fill up it started stalling. Did he use the wrong octane or is there a bigger problem?

Could be another problem. It`s just a coinicidince that the car started stalling. You could by gas at any gas station for this car. It could be cheap gas from a no name station, and the octane level would`t matter. I would be more concerned if he was driving a high end car like Jaguar, Mercedes, Porshe. etc.etc. -where you need a cetain level of octane. Have your son`s car checked by your service shop to narrow down the stalling problem..
Possible stalling conditions--lack of tune-up, vacuum leaks,ignition problems, fuel pressure. These are possible problems just to name a few. Have the car diagnosed..