L.C.
Can you purchase your son's ticket with the miles? I'm wondering if you can do that and then pay for your ticket. I would tell you to call United and ask them if they can waive the phone rep fee, but they are a horrible airline, and it is hard to talk to anyone on the phone who is fluent in English.
Sorry, I guess I wasn't very helpful. But if you can find a cheap enough flight, maybe it's worth it to you to just buy two tickets on Southwest and save those United miles for another trip. Check the flight prices on Tuesday morning. They always seem to be cheaper then. Good luck!