NEDude is 100% correct. Don't bother researching the ADA. Only the ACAA applies to airlines and there is a ton of information available online. It is pretty specific about what can and cannot be done. Helpers are also specifically addressed in the ACAA.
Being old and unable to speak English is not a cause to deny transportation under the ACAA, but if he fell and was injured prior to boarding and there was any doubt as to whether or not he was medically fit to fly, I would think that would be reasonable grounds unless he had a doctor that said otherwise. But I would think that situation would also be outside the scope of the ACAA.
Not sure what the airport police meant by having the Captain "arrest" the passenger.