Severe engine hesitation or stumble

Recently my step son was driving his car when it began to stumble or hesitate very severely to the point that he had to pull over and have us come get him. The car will start and idle fairly well but when you attempt to give it gas, the engine will not accelerate and will stumble and many times stall. I cleaned the throttle body and MAF with appropriate cleaners, changed the fuel filter, the spark plugs and wires but the problem persisted. I also added fresh gas thinking he might have gotten some bad gas, but did not drain the tank. Still no change in how the car runs. The check engine light has not come on and when I check with my OBDII tool, no codes are indicated. I wondered about the fuel pump being the culprit but the OBD tool showed fuel rail pressure of about 40-50 psi with no big drop when I try to give more throttle. I also wondered if the MAF or ECM could be bad, but with no trouble codes to guide me, I am at a loss. Any suggestions on further diagnostic work-up I can preform or is it time to get a mechanic involved who has more specialized diagnostic tools?
Thank you