One time he came back to our garage, where the road is sloping slightly. Because of that he went a little to fast to control himself, he bumped into the garage and fell over. Another time one of his training wheels came loose and he fell over because of that... After those experience there were a few weeks where he did not want to bike at all. In fact he reverted back to his tricycle for some time...
After some convincing I got him to bike again. I told him we were going to work in the sand and put his tools under the binders (a shovel and a rake...). That worked and we went for a little ride. I had to put a hand on his shoulder though...
That went on for a little while and I had to come up with another trick to make him bike without a hand on his shoulder. I sat down a little way down the road and would make a "whoosh" sound as he biked by. Tobias would say "Look papa, I am going really fast!". Once he got used to that he would bike by himself.
Now he is biking by himself and picking up speed and distance every day! So, he got off to a bumpy start, but now he is right on track, or so I think anyway. The most important part? He is loving it now!