score:4
I'd say it's fairly likely. I've never done it at ORD, but I've received gate passes from United at other airports without much in the way of questions. You might want to phrase it as a gate pass "to help my son with a disability" or something similar, as airline staff usually are told not to pry too deeply into the nature of someone's disability.
United has a lot of ticket counters at ORD, so if someone gives you a hard time, you could even try walking to the next bank of counters and asking a different employee there.
You will need photo ID though. Note that if he's arriving on an international flight (not from a preclearance airport), this won't be possible, since he'll have to go through immigration and customs, where you won't be allowed. If this applies to your situation, the airline may be able to provide an escort or other assistance. You can call the accessibility desk at 1-800-228-2744 for specific help from United.