Like his father, Michael Reagan is getting some radio experience. President Reagan's adopted son was a little late starting his morning broadcast on a San Diego radio station Wednesday but he had a good excuse - he had to call his stepmother, Nancy, and wish her a happy birthday. Reagan, who has worked as a speedboat racer and an actor and written a tell-all book about his family, is filling a three-week guest stint on KSDO's morning call-in show. His first week has been busy with the Navy downing an Iranian airliner and Ed Meese quitting as attorney general.