Can you get him to a vet? No open wounds? The swelling could be bleeding under the skin or from the trauma and muscle damage.
He probably needs, at the very least, anti-inflammatories and probably pain meds. Can you get Banamine? If he does have an open wound that you haven't seen yet he would likely need antibiotics. Is he up to date on vaccinations - namely Tetanus? Is he up and around? Eating/drinking?
You can give aspirin for pain and inflammation if that's all you have or all you can get. But, if he is bleeding under the skin it wouldn't be warranted. The dose is one regular aspirin for 10 lbs of body weight. You can crush it and mix it with something sweet and push it through a syringe (no needle of course) with the opening enlarged so it won't stop up.