I personally do not care if he drives a trike, scooter, learner's bike or real bike, as long as he *drives*. There's nothing more annoying than transporting both a kid and the vehicle whose sole purpose was to transport the kid in the first place :-/
Update: My father fine-tuned the bike and they did their first tour around a lake on real bikes.
Fun fact: Training wheels aren't sold much since learner's bikes hit the road (sorry!) a few years ago. Kids aged 3-4 pick up riding real bikes within hours.