Hi OP,
OCD can manifest in this way, as others have said.
In my experience, therapy can be very effective so don't be alarmed by posters saying that it is incurable. With Cognitive Behavioural Therapy as well as Mindfulness for OCD, I have learned very effective strategies to use when these thoughts come up, so in some ways I consider myself cured.
I found out what I needed to know first by reading the book "Overcoming OCD" by Dr David Veale.
Not all GPs or counsellors are knowledgeable about OCD, especially the "harm" type of OCD. Therefore you sometimes need to tell them that Cognitive Behavioural Therapy is the recommended treatment type.
I highly recommend that you ask your GP for a referral to Cognitive Behavioural Therapy NOT any other kind of counselling and that you download and take with you one of the information sheets from OCD Action or OCD UK, which have info on specific kind of OCD including what is sometimes called "Harm" OCD.
This kind of OCD is sometimes called "Pure O" as there are no obvious compulsions occurring (e.g. an obsession with germs leads to compulsive handwashing).
However some say that the "compulsion" in this kind of case is to ask for reassurance or look for reassurance that you have not done somebody harm (so your son may want to go back to where they feel that they have committed a crime or caused an accident, and this is the compulsion).
In my past, I have had well meaning counsellors try to delve into my past life experiences, as well as GPs say that "everyone is a bit OCD" and that "it can't be OCD because you don't have compulsions".
This wasted a lot of time and caused me a lot of worry and grief, and it was only after I did my own research that I was able to push for a referral to a CBT specialist. I didn't reveal the full range of my thoughts until I had been reassured that she had treated harm types of OCD before.
Good luck, and please know that the right kind of help is out there. The sooner you get help for your son the better the outlook will be for him.