"he listens to the radio" "he listens to a radio" is I think a little less common. you would say this to mean somebody "listens to a radio" but "to the radio" equals the same and is much more natural. "he listens to the radio all day" = any radio or a particular radio. The radio in his room for example. "he listens to a radio all day" = suggests any radio so long as it is a radio, but it could also be a specific radio. That's why "he listens to the radio" is used more frequently

He listens to the radio

"He listens to radio" is perfectly correct. "Radio" has more than one meaning. It can be the medium. For example, "radio forces us to use our imagination, while television does not." You could also say "I love radio," or "I hardly ever listen to radio now that we have podcasts." "Radio" can also be short for "a radio receiver." Thus you could actually say "I like to listen to radio. In the house, I listen to radio on a little radio in the kitchen, and in the car I listen to radio on the car radio."

Does he listen to podcasts? No he listens mostly to radio. . Not impossible. It means "radio" as a generic medium, contrasted with other things. . As the other say "the radio" meaning the device, is much more common a meaning to need, so much more common to say or hear.