I reread your post, I only suggested looking up allergies because I thought you implied your dog had them…..when you mentioned an otc allergy med.
I would refrain from giving this dog any supplements or over the counter meds unless recommended by a veterinarian that has examined him.
It really sounds like his problems are related to an injury he may have obtained during that accident, especially if that is when his symptoms started.
It’s important to get him examined and diagnosed by a vet, he sounds uncomfortable, some dogs are stoic and try to hide it when they are in pain.
