I see - personally I would not fly an airline that has only one flight per day, I fly often and 1 out of about every 5/6 times I fly there is an issue. Most recently we were onboard an aircraft and then there was an issue with the flight crew, since it was a legacy airline I just went to the nearest kiosk, changed my flight and arrived in time for my plans.
I would take a layover vs frontier or spirit but that's just my opinion - lots of people never have an issue! For me I don't want to waste a day waiting on a new flight in either direction and for me one flight a day isn't an option.
Lots of people never have an issue with these airlines either so I agree with the pp to let your son make the decision. Nothing like giving advice and then having something go wrong.... It happened to me once when I booked a flight for my mil and fil - you would have thought I was the cause of the delay
My dd does like frontier - she flies alone so she doesn't care where she sits, has a flexible schedule and can get by with carryon!
