What are applicable laws?
The rules are spelled out in Spirit's Contract of Carriage (https://www.spirit.com/Content/Documents/en-US/Contract_of_Carriage.pdf) and terms and condition (https://www.spirit.com/Legal.aspx), which you agreed to at time of booking.
The contract of carriage says in section 8.2
Rebooking When a guest holding a confirmed reservation on a Spirit
flight which is delayed because of a schedule irregularity (including
but not limited to, a missed connection, flight cancellation, omission
of a scheduled stop, substitution of equipment, or schedule change),
Spirit may rebook the guest on Spirit's first flight on which seats
are available to the guest's original destination without additional
charge. Our staff will focus on rebooking as many guests as possible
on alternate flights, either direct to the destination or via
connections through other airports to best accommodate the guest’s
needs. A change may be made to an itinerary without a charge and/or
fare difference when the itinerary was affected by a cancelled flight,
an eligible schedule change or a delayed flight (greater than two
hours from the original departure time) provided:
- The same departure and arrival airports are booked and;
- The itinerary is rebooked within Spirit’s authorized date ranges (currently within 7 days of the departure date).
With limited exceptions, Spirit will not reimburse
guests for flights that they book on other carriers.
That's unfortunately open to interpretation, so I think it's probably more important right now to figure out how to get your son to Atlanta most efficiently. Today looks pretty bleak, but tomorrow there are non-stop flights on Spirit and Jet Blue for $200-$250. It's puzzling that Spirit claims "no seats available". Check online booking sites and document any available Sprit flights. If they have open seats, they need to give them to your son. If they don't, they should rebook him on Jet Blue. Now it's time to start a solid paper trail.