Definitely Medicare will not pay for air ambulance for this circumstance, as has been clearly stated above.
I have read a few stories on this board of people fighting to get their private medical insurance to pay for transport to out of state rehabs like Shepherd, and winning after extensive appeals/letter writing and doctors helping them. But Medicare will never help. I do not recommend trying to appeal to Medicare. Save your energy and look elsewhere.
But do get familiar with the details of Medicare coverage, and the limits for number of days in the hospital per year and per lifetime. You need to get him to rehab as soon as he is medicinally stable so that those precious days are used in rehab.
We were in the same situation as the OP. Parents stuck out of state, no one cares about getting you back home. And when you are older, they care even less.... And when you are older with a spinal cord injury.....