He needs to preboard and they should stow the walker in a closet or overhead. He won't be able to use the walker onboard due to space limitations in the aisle. I would also suggest strongly that you call on his behalf and see if AA would rebook him on an earlier flight from RSW because that connection is beyond risky. As he's flying international F he will be able to use the extra time to relax and enjoy the lounge.
If, for example his flight comes into one terminal and leaves from another, and he has to walk to a train, take an elevator to the platform and then down at the arriving station, and then reach his connecting gate, he may not be present by the required cut off in boarding and be denied. Then what? It is not worth it. I can run between flights and I'd never risk it, never mind a walker. Put that in his head and see if he reconsiders.
I suspect if you call AA and plead your case they will reaccomodate him on the earlier flight. And, if there isn't one, go a day earlier and stay at the Hyatt at the airport. Peace of mind is worth the cost.