It's exposure therapy and I doubt whether a squint would effect that. He can see and that is the main thing.
EMDR can be very effective, although it's not a quick fix, but I think you are aware of that already.
The positive will be if your son agrees to not self harm, it's like making a deal. If he agrees, it will show he is willing to get the help he needs.
It must have been an exhausting day for you both and you had storm Brendan to deal with too.
Pleased you both back safely. xx