Both mean the same thing. In context, one could be referring to permission and another to ability. But either could be used for permission and either could be used for ability.
Possibly, in context, the first could mean that the person only has permission or ability (whichever it is) to drive one car, while the second means he has permission or ability to drive many cars. I'm hard pressed to think of an example, though, where we'd say "a car". I mean, you might say, "George can drive this car", and indicate one particular car that George is allowed to drive, while he is not allowed to drive any of the other cars.
Normally, a person can only drive one car at a time. So if we are talking about ability, whether you use singular or plural, you are saying that he knows how to drive. If we were talking about a particular trip, we would use the singular, because he's only going to drive one. Like, "We'll need two cars to get us all there. George can drive a car, and Bob can drive another one." I suppose if the trip involved drivers switching car, you'd say "can drive cars".