I'm very sorry to hear about your relative's injury.
You've been given good advice about sources of info about the accident. Police, fire or EMT reports may also list witnesses and their contact information, if any was obtained, and hospital intake information will reflect the account of the accident that was given when he was brought into the ER.
If you are thinking of placing a claim against the city (which still operates ORD), having a local lawyer's staff do the investigative work for you may be more efficient and productive than a post here on FT, and worth the investment.
Finally, I think you'll find that all the stairs at ORD have handrails.
But so sorry about the accident.