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It sounds like your pet has suffered a severe blow to the head. He might have a concussion or other head injury that could be fatal.
There is not much that you can do for this at home. I would keep him quiet through the night and see the vet as soon as you can, tonight if you can afford it.
Let me know if you still have specific questions. I fear that my answer has been too broad.
I can't tell you to give aspirin for pain because it has a blood thinning effect and if he has a brain bleed, this would make it worse....