I don't think there's any intent to actually make him a spacecraft pilot. NASA's been making use of doctor-astronauts for decades, and the astronaut corps in general does a lot more than just get ready for their next flight. If they're awaiting a flight assignment, NASA finds something for them to do (or they find themselves something to do) that dovetails with their training and background. The MDs often get involved with designing on-orbit medical tests or experiments, or help conduct them from the ground, or train the other astronauts in medical procedures, for example. The Air Force has "pilot-physicians" and probably an AF astronaut brought the idea into the program. In this case, I would speculate that there was a need or vacancy for a pilot-qualified MD within the office and he put his hand up.
(not to say it's out of a sense of team spirit alone...often they volunteer for assignments they think might also make them more competitive for an upcoming crew assignment)
I've been reading
Homesteading Space, about the Skylab program, co-written by Joe Kerwin, and there was a lot of interesting detail about how the astronauts spend their time between or before their flights.