Have your son hold the accelerator to the floor, turn the key to start and hold for a few seconds. You be at the back and feel for exhaust while it is cranking. You should feel pulses of exhaust. If you don't feel any exhaust or it is a small amount and very smooth (no pulses) then the exhaust is plugged. Exhaust plugged = no start.
It took me 2 days to find a plugged exhaust the first time saw it. That was back in the late '70s.
For me, 1. Check for fire at plugs. 2. Check for and/or add fuel to intake. 3. Check for plugged exhaust. 4. Check compression.
Your compression is good enough for the engine to run and once started I'm sure it will even out.
An engine needs fire, fuel, compression, exhaust all at the correct time and it will run.

Paul

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84 500 SEL (307,xxx miles)