There is a passenger assistance category which covers it, check him in at the assistance desk and have them guide him to the departure lounge and gate.
As for her US visa, if I had the option of going to Belfast, Dublin, Amsterdam or Paris to get mine, then surely Lundun or Belfast are options.
Also for his ESTA, he needs to be sure to buy it directly from the the US.gov site and not from a Google search.
https://esta.cbp.dhs.gov/esta It only costs a few bucks, so anything over $21 is essentially a rip off and will take longer, although he needs to be sure that he has the correct visa for his trip.
The US embassy appointments start off months away, but if you keep checking the appointment site at obscure times, you can literally shave months off the queue. Mine went from nine weeks down to about eight days.