You would have to call the airline and ask for a gate pass, however this varies from airport to airport and carrier to carrier. You may only be able to escort him to TSA where they will have someone escort him (if necessary in a wheelchair or other form of transportation as needed) to the gate and onto the airplane. Upon arrival there will be someone from the airline waiting for him and will escort him to whoever is picking him up. He would not be alone if that is your concern. They would have this tracked in the system and they will be waiting for him on both ends of the flight.
Some airlines issue gate passes, but it varies from airport to airport. Call airlines.
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