Here is another German father/son story called "Kleine Kreise" in its own language. It appears to have been released in 2001, but made a few years before that.  Jacob Matschenz is the boy actor of interest. The synopsis again comes from Kino magazine.

A notorious initiator of unsuccessful business ventures, Paul lives in a garage between a trading estate and a furniture centre. Since his wife left him, he has not seen his son Niki very often. One day during a weekend visit, the two of them get hold of a racing go-cart, just to take a few rides around. They quickly develop a taste for it. Together with the grumpy shopkeeper Karl, whom they exploit as a sponsor, they set out to roam from race to race during the summer holidays. It soon becomes fairly obvious that they don't have a chance. But Paul is far too stubborn to give up easily.
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Does anyone know how old the 'boy' is supposed to be?

Admittedly he's good looking in a gothic kind of way but he looks over 15 to me (his 'Depardieu' proportioned honker is a giveaway) so I'm not going to create a profile page unless there is evidence that he's 15 or under.

The boy was 15 when the movie was made and his character had the age of fourteen. I have seen this movie on TV and was not that impressed about it.

'soles'