Yes, you can request wheelchair assistance online with AA. DEFINITELY do that ahead of time. Be advised that my personal experience with wheelchair assistance on AA in Miami has been spotty at best. Be prepared for waiting long periods of time. The arrival back into Miami is particularly difficult. The last time we went through there, we walked down the hallway into the terminal and waited for a wheelchair, which took us to a place where we waited for a motorized trolley to take us for a bit. Then we were dumped off that and waited AGAIN for a wheelchair, to go the rest of the way. It was well over an hour from when we deplaned until we got our luggage (which was already waiting for us when we arrived in baggage claim), and the VAST majority of that time was consumed in just GETTING to the line for immigration. We have not dealt with a connection in Miami, but based on our experience, I hope you have a pretty good amount of connection time.

With regard to the stairs in SXM, MOST of the time, the AA flight from Miami to SXM and the return will have a RAMP at the FRONT of the plane, and stairs at the back. If your seats are at the back of the plane, suggest that you wait until almost everyone has deplaned, and deplane out the front of the plane, so that he can use the ramp. There will be a person with a sign with his name, waiting on the tarmac, or close to the bus, if you are taking the bus to the terminal. They will assist him with getting on the bus and getting him through immigration and help him get his checked luggage, if any. When you are leaving, the folks there will try to steer him to the back if his seat is in the back. Just inform the person that your dad needs to use the ramp. Also, check with the gate people in SXM when you are leaving, to see if they will let him pre-board the plane, as they usually allow that. The people in SXM do a wonderful job.

Please let me know if you have any other questions.