My son owns a 1994 Eldorado. He went to fill up at the gas station and the attendant stopped at $2.50 because gas was leaking all over? What could this be?

You may have leakings fuel line on top of gas tank when filling up or line is leaking where they connect to fuel filter under body maybe behind rear wheel wells , or may have a leaking furl injector or even your carbon gas canister filtr need chaging or ovver filling can send fuel up to that canister which recirculates gas fumes back to fuel system check your gas cap maybe leaking or when gas attended over filling it runs down into drain tube

Joe

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..

no regular n premuim will not stop the car from running

This is caused by some gas station pumps, not your vehicle. The sensor on a lot of pumps at stations are not replaced or adjusted, or the nozzle is not pressed in far enough when the attendant inserts the nozzle when fueling a vehicle. I too had the same problem, and realized the solution after testing other fuel stations/pumps.

The gas filter is mounted on the rear area of the frame near the tank

You can use 89 octane. Cadillac wanted all customers think a high end car takes high end fuel which is untrue. I am retired from cadillac too

disconnect and reconnect the - battery cable. 

run the fault codes to check for EVAP system not venting properly

I would make sure it is leaking externally. That amount you should be able to tell if it is leaking or not. If it is not it is most likely a blown head gasket and you are loosing the fluid internally. 

it could be two things: the hose that is connected to the top of the tank has a hole in it or the tank itself has a hole