A very interesting thread this is.
The usual way to deal with a soul that is stuck is to tell them to go into the light.
Souls become stuck because they fear judgement and therefore fear going towards the white light that one encounters on the other side - or because they died suddenly, as in this case by way of an accident. When they died so suddenly they might not even have realized that they have died. They might indeed relive the accident again and again, each time trying to avoid the accident - without succeeding.
You say the car involved has brought tragedy to anyone who touched it. Such things are indeed not unheard of - the car that the actor James Dean died in had a similar history. Cars have their own spirits too, everything has a spirit.
What happened with your son (him choosing a toy car that was similar to the one this tormented soul died in) would be your son picking up on this sad story. You both indeed seem to be empaths, as pointed out by others on here. Therefore you sense what is going on and what this tormented soul is going through.
As an empath myself, I get the strong urge to tell you to tell this soul to go into the light. You say the location is at the other side of the country - sometimes however it needs someone who can sense what is going on to deal with such situations.
I have to admit though, it's not something I would want to do. So if you are not comfortable with this - try and get someone else to do it, or just pray for this lost soul.
Also ensure that whatever you do, you protect yourself and your son. You say you don't think this lost soul is an evil spirit - but you never know because evil spirits are deceptive. You can sense the true nature of this spirit only if you really tune in.
ANKH EM MA'AT