I seriously hope you will take your dog to the vet as soon as possible, he may have obtained some type of internal injury due to the accident you described, that only an x-ray would reveal.
I think it would be money well spent, rather than buying supplements and over the counter meds (not prescribed by a vet) that may not help and may cause more harm.
And trying various different brands of food, his issues may have nothing to do with the food.
Use the search engine here, look up allergies. I would trust a vet that has examined my dog. My dogs enjoy going to the vet.
Best of luck.
