When you say reserved, do you mean that he has already purchased the ticket? If so, and it is on a traditional major carrier, it will cost him $100 to change or cancel it. SWA and JetBlue don't charge those big change fees, which is why they are ideal in situations like this, where plans may have to change at the last minute.
Though Southwest and JetBlue airlines go only into MCO, in addition to Melbourne, there is also Daytona Intl. nearby; and there are direct flights on Continental from Newark into Daytona. It's a very small airport, and it is a pretty straight shot from Daytona down to Cocoa if he follows the coast road (US1) no interstate driving required, though I-95 parallels US1. If driving itself isn't the issue, he might be able to manage a rental from there. (Though you should arrange it for him in advance and have all the info printed out. Then tell him to insist that he wants exactly what is printed on the paper he has; no extra-charge upgrades or extra insurances.)
A car service from MCO would probably be cheaper than a cab; cabs have meters, but car services do not use them, so traffic will not run it up. A car service might even let you book a r/t from the airport to Cocoa for him with an open return date, if you explain the situation.
I suppose there is no chance that another relative might accompany him down and stay for a couple of days to get him acclimated? We find that to be the best course when older first-time flyers have to go somewhere in an emergency. They are usually OK on the way home, but it helps to have an experienced air traveler show them the ropes on the way down, especially since they tend have their minds wound up worrying about the sick relative.
You FIL should be on notice that if he is flying on a one-way ticket, he will be subjected to extra security screening by TSA. He is almost guaranteed to have his carryon searched by hand, and he may be wanded and patted down. You should probably go to
http://www.tsa.gov/public/display?theme=175 and print out some of the guidelines and FAQ's for him, so that he will know what to expect going through security.