1 hour 10 is do-able at PHL, but leaves very little room for delays, F and A are the furthest apart in the airport. There is a bus linking Terminal F (Gate F10 if I remember) and Terminal A (Gate A1- Terminal A-East).

To put this in perspective, in 2007, I arrived at F on a very badly delayed YUL flight and still made in on to a DUB from A-West, they literally closed the doors behind me. From they opened the doors at F I had 25 minutes until pushback at A-West, my luggage of course never made it across the pond until the next day.

If I was your son, I'd leave the connection the way it is but ensure there is a fallback US to CUN for later in the day. US seem to be masters at squirming out of providing hotels for misconnecting pax.